Babesia bigemina

Babesia bigemina is a tick-borne intra-erythrocytic protozoan (Phylum Apicomplexa, Order Piroplasmida) causing bovine babesiosis (redwater fever, Texas fever). The vector is the one-host cattle tick Rhipicephalus (Boophilus) microplus.

Life cycle (digenetic)

Two hosts: cattle (vertebrate) and the tick (invertebrate).

In cattle

1. Sporozoites are injected with tick saliva and enter red blood cells.
2. Inside the RBC they become trophozoites, which divide by binary fission/budding (merogony) into pairs of pear-shaped (pyriform) merozoites, 2.5-5 µm, joined at an acute angle.
3. The RBC ruptures, releasing merozoites that invade fresh cells. Some merozoites develop into gametocytes (precursors of gametes).

In the tick

4. A tick feeding on infected blood ingests the parasites. Gametocytes form gametes in the gut, which fuse to form a zygote and then a motile ookinete (kinete).
5. Kinetes pass from the gut into the haemolymph and invade many organs, including the ovary, so the parasite is passed transovarially to the eggs and the next tick generation (in B. bovis the parasite likewise passes transovarially, but is carried to the host by larvae).
6. Sporogony in the salivary glands of the larval/nymphal tick produces infective sporozoites, which are injected into the next host.

Pathogenicity

• The parasite multiplies in and destroys RBCs, causing haemolytic anaemia, haemoglobinaemia and haemoglobinuria (dark red "coffee-coloured" urine - hence "redwater"), and jaundice.
• Incubation period 8-15 days. Signs: high fever (40-41.5 °C), depression, loss of appetite, pale or yellowish mucous membranes, rapid pulse and breathing, weakness, constipation followed by diarrhoea, abortion in pregnant cows.
• Untreated acute cases may die in 3-7 days, mortality in susceptible adult cattle up to 50-90%.
• Post-mortem: enlarged, pale and jaundiced liver, enlarged spleen, congested kidneys, watery blood, petechiae.

Control

1. Vector control: regular dipping or spraying with acaricides (amitraz, cypermethrin, deltamethrin), rotational grazing and pasture spelling, hand-picking and inspection of animals.
2. Treatment: diminazene aceturate (3.5 mg/kg intramuscularly, drug of choice) or imidocarb (1.2 mg/kg subcutaneously); supportive iron, fluids, and blood transfusion.
3. Immunisation: live attenuated vaccines (premunition), used in endemic areas.
4. Management: quarantine and tick-free introduction of animals, tick-resistant breeds, and awareness among farmers.

Structure and Life Cycle of Schistosoma haematobium

Systematic Position

Schistosoma haematobium is the blood fluke that causes urinary schistosomiasis (bilharziasis) in humans. It is dioecious (separate sexes), unlike most trematodes which are hermaphrodite.

Structure

Adult Worm

• Size: Male 10–15 mm long; female 15–20 mm long (longer but more slender).
• Body: Cylindrical, unsegmented, soft, and covered with a spiny tegument (cuticle bearing tubercles/spines).
• Suckers: Two suckers — an anterior oral sucker surrounding the mouth and a ventral sucker (acetabulum) for attachment to the host's venous wall. No pharynx is present.
• Gynaecophoric canal: The male body is folded ventrally to form a longitudinal groove called the gynaecophoric canal, in which the female lies permanently (hence the name "blood fluke" — they live in pairs in blood vessels).
• Digestive system: Mouth → short oesophagus → bifurcated intestine (two caeca) that reunites posteriorly into a single tube. No anus — undigested matter is regurgitated.
• Excretory system: A pair of flame cells opening into collecting tubules that unite to form a bladder and a single excretory pore at the posterior end.
• Nervous system: A pair of cerebral ganglia with longitudinal nerve cords.
• Reproductive system:
  • Male: 4–5 testes (in S. haematobium) lying behind the ventral sucker, opening into a common vas deferens and a cirrus.
  • Female: A single ovary, a short oviduct, a vitelline gland (yolk gland), and a uterus opening at the genital pore situated just behind the ventral sucker. The uterus contains 20–30 eggs at a time.
• Absence of: adults lack a muscular pharynx and an anus.

Egg

• Oval, terminal-spined (a prominent spine at one pole), measuring about 110–170 µm × 40–70 µm.
• Contains a miracidium (ciliated larva) when passed in urine.
• The spine helps the egg to anchor in the venule wall and later to rupture into the bladder lumen.

Life Cycle

S. haematobium is digenetic (requires two hosts): the definitive host is man and the intermediate host is a freshwater snail of the genus Bulinus.

Steps

1. Egg in urine: The gravid female migrates from the pelvic veins to the venules of the urinary bladder and ureters, where it lays eggs. Eggs pass through the bladder wall into the lumen and are excreted in urine (hence "urinary schistosomiasis"). Eggs may also be passed in faeces in light infections.

2. Hatching (Miracidium): In fresh water, the egg hatches within 15–30 minutes, releasing a miracidium — a ciliated, pear-shaped, free-swimming larva.

3. Penetration into snail: The miracidium swims actively and penetrates the soft tissues of the intermediate host snail (Bulinus species) within 24 hours.

4. Sporocyst stages in snail:

   • Miracidium loses its cilia and becomes a mother sporocyst.
   • Mother sporocyst produces daughter sporocysts (asexual multiplication).
   • Daughter sporocysts produce cercariae (the infective larval stage).

5. Cercaria (free-swimming larva): The cercaria has a bifurcated tail (furcocercous) and a head gland. It escapes from the snail into water and swims actively.

6. Penetration into man: The cercaria penetrates the unbroken skin of a human (usually while wading, bathing, or swimming in contaminated water). It loses its tail during penetration and becomes a schistosomulum.

7. Migration in human host:

   • Schistosomulum enters the lymphatic/blood vessels → right side of the heart → lungs → left side of the heart → aorta → hepatic portal system.
   • In the liver, the worms mature, pair up (male and female), and migrate through the portal veins to the vesical and pelvic venous plexuses.

8. Adult worms in venules: The paired worms reside in the vesical veins around the urinary bladder. The female lays eggs, which either pass out in urine or are carried by the bloodstream to other organs (liver, lungs, etc.), causing granulomatous lesions.

9. Completion of cycle: Eggs reaching fresh water hatch into miracidia, and the cycle repeats.

Diagrammatic Summary

Adult pair in vesical veins
        ↓ (eggs in urine)
Egg (terminal-spined) in water
        ↓ (hatching)
Miracidium (ciliated)
        ↓ (penetrates snail)
Mother sporocyst → Daughter sporocyst → Cercaria
        ↓ (escapes snail)
Cercaria (bifurcated tail) in water
        ↓ (penetrates human skin)
Schistosomulum → heart → lungs → liver → vesical veins
        ↓
Adult pair (male + female)

Pathogenicity and Symptoms

• Cercarial dermatitis (swimmer's itch): itching and rash at the site of penetration.
• Katayama fever: acute allergic reaction during maturation (fever, chills, cough, eosinophilia).
• Chronic stage: haematuria (blood in urine), dysuria, frequency, and terminal haematuria — the hallmark of S. haematobium infection.
• Complications: bladder fibrosis, hydronephrosis, squamous cell carcinoma of the bladder, and pulmonary hypertension.

Prevention and Control

1. Avoid bathing or wading in contaminated fresh water.
2. Treat infected persons with praziquantel (drug of choice).
3. Control snail population (molluscicides, biological control).
4. Improve sanitation — avoid urinating in or near water bodies.
5. Supply safe drinking water and health education.

Conclusion

Schistosoma haematobium is a dioecious blood fluke with a terminal-spined egg, living as a pair in the vesical veins of man. Its life cycle requires man as definitive host and freshwater snail (Bulinus) as intermediate host, with miracidium, sporocyst, and furcocercous cercaria as larval stages. It causes urinary schistosomiasis, characterised by haematuria and bladder complications.

Leech (Hirudinaria granulosa) is a hermaphrodite animal, i.e., both male and female reproductive organs are present in the same individual. The reproductive organs are well developed and occupy a considerable part of the body cavity.

Male reproductive organs

1. Testes — There are 11 pairs of small, rounded, lobed testes. They are located in the middle of the body, one pair in each segment from the 12th to the 22nd segment, lying ventro-laterally between the crop caeca.
2. Vasa efferentia — From each testis arises a short duct called vas efferens. The vasa efferentia of each side unite to form a longitudinal duct, the vas deferens.
3. Vas deferens — Two long, thin, coiled tubes (one on each side) run forwards and open into the prostate gland.
4. Prostate glands — A pair of elongated, glandular, lobed structures lying in the 18th and 19th segments. They secrete a fluid that nourishes and activates the sperm.
5. Ejaculatory ducts — Each prostate gland continues forward as a short ejaculatory duct.
6. Atrium and male gonopore — The two ejaculatory ducts open into a muscular chamber, the atrium (or genital atrium), which opens to the exterior through the male gonopore situated mid-ventrally in the 10th segment.

Female reproductive organs

1. Ovaries — There is one pair of ovaries, located in the 11th segment, one on each side of the ventral nerve cord. Each ovary is a small, rounded, lobed structure.
2. Oviducts — From each ovary arises a short, ciliated duct called the oviduct. The two oviducts run downwards and unite to form a short common oviduct.
3. Vagina and female gonopore — The common oviduct opens into a small sac, the vagina, which opens to the exterior through the female gonopore situated mid-ventrally in the 14th segment.

Accessory reproductive structures

• Clitellum — A glandular, thickened region covering segments 10 to 12. It secretes the cocoon (egg case) in which fertilized eggs are deposited.
• Albumen glands — A pair of glandular structures opening into the female genital duct; they secrete albumen which nourishes the developing eggs.

Fertilization is cross-fertilization and mutual — two leeches in copulation exchange spermatozoa, since self-fertilization is prevented by the positions of the male and female gonopores (male at segment 10, female at segment 14). The sperm received is stored in the vagina, and fertilization occurs when eggs are laid in the cocoon.

Pincer and Sucking Type of Mouth Parts

1. Pincer Type of Mouth Parts (Chewing and Biting Type)

This type is found in insects such as cockroach, grasshopper, and beetle. The mouth parts are hard, toothed, and adapted for biting and crushing food. The following parts are present:

(a) Labrum (Upper lip): A broad, flat, sclerotized plate attached to the clypeus. It forms the roof of the mouth cavity and helps in holding the food.

(b) Mandibles (Upper jaws): A pair of hard, strongly toothed, chitinous structures. They are the main biting organs. Each mandible has an inner toothed (molar) region for crushing and an outer sharp (incisor) region for cutting the food. They move transversely (side to side).

(c) Maxillae (Lower jaws): A pair of segmented appendages lying below the mandibles. Each maxilla consists of:

• Cardo – basal segment
• Stipes – middle segment
• Lacinia – inner hard, toothed lobe for holding food
• Galea – outer soft lobe
• Palp – a five-segmented maxillary palp for sensory (tactile) function

(d) Labium (Lower lip): A fused, shield-like plate forming the floor of the mouth. It bears a pair of three-segmented labial palps and supports the other mouth parts from below.

(e) Hypopharynx: A tongue-like structure lying between the mandibles and labium. It bears the opening of the salivary duct and helps in swallowing.

2. Sucking Type of Mouth Parts (Piercing and Sucking Type)

This type is found in insects such as mosquito, bug, aphid, and butterfly. The mouth parts are modified into a long, slender proboscis or beak for piercing the host tissue and sucking liquid food (blood or plant sap). The parts are as follows:

(a) Labrum: In mosquito it is long and needle-like and forms the roof of the food canal; in butterfly it is reduced or absent.

(b) Mandibles: In piercing-sucking insects (mosquito, bug) the mandibles are modified into long, slender, sharp stylets that pierce the skin of the host. In butterfly they are absent.

(c) Maxillae: The maxillae are the most important piercing organs. The laciniae are drawn out into long, sharp stylets that form the piercing needle. In butterfly, the galeae are greatly elongated and interlocked to form a coiled sucking tube (proboscis) for sucking nectar. The maxillary palps are reduced or absent.

(d) Labium: The labium forms a thick, grooved sheath (rostrum) that encloses and protects the stylets. In mosquito it does not pierce but guides the stylets; in bug it is jointed and forms the beak. In butterfly it is reduced to small palps.

(e) Hypopharynx: It is a long, sharp stylet lying between the mandibles and labium. It contains the salivary duct, through which saliva is injected into the host to prevent blood coagulation and to aid sucking.

Summary: In pincer type the parts are hard and toothed for biting and chewing solid food, whereas in sucking type the parts are elongated into needle-like stylets and a proboscis for piercing tissues and sucking liquid food.

Water Vascular System in Asterias

The water vascular system is a unique echinoderm feature that functions in locomotion, food capture, respiration and excretion. In Asterias (common sea star) it is a complex system of canals and tubes filled with sea water.

Components:

1. Madreporite (sieve plate): A perforated calcareous plate on the aboral (dorsal) surface, slightly to one side of the inter-radius. Sea water enters through its grooves and pores.

2. Stone canal: A short, S-shaped, calcareous tube connecting the madreporite to the ring canal. Its wall is strengthened by calcareous rings.

3. Ring canal: A circular canal lying in the perioral coelom around the oesophagus, just above the mouth. It is connected to the stone canal and gives off five radial canals and paired Tiedemann's bodies and Polian vesicles.

4. Radial canals: Five radial canals arise from the ring canal and run along the ambulacral groove of each arm up to the tip. Each gives off lateral branches (lateral canals) to the tube feet.

5. Lateral canals: Short branches given off from each radial canal; each lateral canal bears a valve and opens into a tube foot.

6. Tube feet (podia): Each lateral branch ends in a tube foot consisting of:

   • Ampulla — a small muscular bulb above the ambulacral ossicle.
   • Podium (stem) — a hollow tube passing through the ambulacral groove.
   • Sucker (disc) — a terminal cup used for attachment.

7. Tiedemann's bodies and Polian vesicles: Small sacs attached to the ring canal; the Polian vesicles store water and the Tiedemann's bodies produce coelomic fluid.

Working:

• Water enters through the madreporite → stone canal → ring canal → radial canals → lateral canals → ampullae.
• Contraction of the ampulla forces water into the podium, extending the tube foot; the sucker attaches to the substratum.
• Relaxation of the ampulla allows water to return, shortening the tube foot and pulling the body forward.
• Valves in the lateral canals prevent backflow, so the tube feet act as a hydraulic system for slow creeping locomotion.
• The system also helps in gripping prey (e.g. opening bivalve shells), respiration (gas exchange across thin podial walls) and excretion of nitrogenous wastes.

Significance: The water vascular system is the chief organ of locomotion in Asterias and is a characteristic feature of the phylum Echinodermata.

Affinities of Branchiostoma (Amphioxus)

Branchiostoma (commonly called Amphioxus or lancelet) is a small, fish-like, marine chordate that burrows in sand in shallow seas. Its affinities are discussed under three heads: affinities with invertebrates, affinities with vertebrates, and its systematic position.

1. Affinities with Invertebrates

Branchiostoma shows several features resembling invertebrates, which is why it was once placed among molluscs and later among annelids:

• Shape and habits: The elongated, laterally compressed, fish-like body and burrowing habit resemble annelids and some molluscs.
• Ciliary feeding: Cilia lining the pharynx create a water current for feeding, as in many invertebrates.
• Segmental structures: The segmental arrangement of myotomes, gonads, and nerves resembles metamerism of annelids.
• Nephridia: The excretory organs (protonephridia with solenocytes) resemble those of annelids and flatworms.
• Absence of jaws and paired limbs: Like invertebrates, it lacks jaws and paired appendages.
• Blood vascular system: It has no true heart; blood is colourless and flows in closed vessels without a definite pump, as in many invertebrates.
• Larval features: The larva resembles the trochophore-like larva of some invertebrates in its ciliary bands.

However, these resemblances are superficial or due to convergent evolution, and do not indicate true invertebrate affinity.

2. Affinities with Vertebrates

Branchiostoma shows striking resemblance to vertebrates (especially the cyclostomes) and is therefore regarded as a primitive chordate:

• Notochord: A persistent notochord extends the whole length of the body and persists throughout life — a chordate character.
• Dorsal hollow nerve cord: The central nervous system is a dorsal, hollow, tubular nerve cord, as in vertebrates.
• Pharyngeal gill slits: The pharynx is perforated by numerous gill slits supported by gill bars, as in fishes.
• Post-anal tail: The tail extends beyond the anus and contains the notochord and nerve cord.
• Coelom and mesoderm: It has a true coelom and triploblastic, bilaterally symmetrical body with organ-system grade of organisation.
• Muscle blocks: The body muscles are arranged as segmental myotomes, as in fishes.
• Endostyle: The endostyle in the pharynx is homologous to the thyroid gland of vertebrates.
• Hepatic caecum: The hepatic caecum is regarded as homologous to the liver of vertebrates.
• Nervous system: The brain is not well developed, but the nerve cord shows a slight anterior swelling (cerebral vesicle) and gives off nerves as in vertebrates.
• Embryology: Development shows a hollow blastula, gastrula, and a ciliated larva resembling the tadpole larva of ascidians and the ammocoete larva of lampreys.

3. Differences from Vertebrates (Why it is not a Vertebrate)

• No cranium or brain case, hence no true brain.
• No vertebral column; notochord persists and is not replaced by vertebrae.
• No jaws, no paired fins or limbs.
• No true heart; no chambered heart.
• No specialised kidneys (has protonephridia instead).
• No myelinated nerves; nerve cord is not enclosed in a bony or cartilaginous sheath.
• Skin is naked and single-layered, without scales or keratin.

4. Systematic Position

Because of its dual affinities, Branchiostoma is placed in:

• Phylum: Chordata
• Subphylum: Cephalochordata (Acrania)
• Class: Leptocardii
• Order: Amphioxiformes
• Genus: Branchiostoma

It is regarded as a primitive chordate that retains chordate characters throughout life but lacks the vertebrate specialisations. It is often called a "living fossil" and serves as a connecting link between invertebrates and vertebrates.

Conclusion

Branchiostoma shows superficial resemblances to invertebrates (annelids, molluscs) but possesses all the fundamental chordate characters — notochord, dorsal hollow nerve cord, pharyngeal gill slits, and post-anal tail. It lacks the specialisations of true vertebrates (cranium, vertebral column, jaws, paired limbs, true heart). Hence it is placed in the subphylum Cephalochordata (Acrania) as a primitive chordate, forming a link between invertebrates and vertebrates.

Biological Species Concept (BSC)

Definition: The biological species concept defines a species as a group of actually or potentially interbreeding natural populations that are reproductively isolated from other such groups. It was proposed by Ernst Mayr (1942) and is the most widely used species concept in zoology.

Key criteria:

1. Interbreeding: Members of the same species can mate freely with one another under natural conditions and produce fertile offspring.
2. Gene flow: There is exchange of genes (gene flow) among populations of the same species, so they share a common gene pool.
3. Reproductive isolation: Members of one species are reproductively isolated from members of other species by barriers that prevent interbreeding, so gene flow between species is absent or negligible.
4. Natural populations: The concept applies to populations in nature, not to organisms in captivity or to asexual forms.

Reproductive isolating mechanisms that maintain the species boundary are of two types:

• Prezygotic barriers (prevent fertilization): ecological, temporal, behavioural, mechanical and gametic isolation.
• Postzygotic barriers (act after fertilization): hybrid inviability, hybrid sterility and hybrid breakdown.

Example: The horse (Equus caballus) and the donkey (Equus asinus) are separate species because they do not interbreed in nature; when crossed artificially they give a sterile hybrid, the mule. In contrast, all breeds of dog (Canis lupus familiaris) interbreed freely and belong to one species.

Merits:

• It is objective and testable, based on reproductive criteria rather than on appearance.
• It explains why members of a species resemble one another (shared gene pool) and why species remain distinct (reproductive isolation).
• It gives a clear genetic basis to the species category.

Limitations:

• It cannot be applied to asexually reproducing organisms (bacteria, many protists, some plants) where interbreeding does not occur.
• It is not applicable to fossil species, since their reproductive behaviour cannot be observed.
• It fails for allopatric populations that do not overlap geographically, so their potential to interbreed cannot be tested.
• Hybridization occurs in nature in some groups (e.g., many plants), which the concept does not explain well.

Conclusion: Despite its limitations, the biological species concept remains the most satisfactory and widely accepted concept in taxonomy, especially for sexually reproducing animals, because it defines species on the basis of reproductive isolation and a common gene pool.

Protozoa vs. Metazoa

In short: Protozoa are unicellular, microscopic, organelle-level animals without true tissues, while metazoa are multicellular animals with tissue-, organ- and system-level organization.

Human intrusion in coral reefs refers to the direct and indirect interference by humans that damages reef ecosystems. Coral reefs are among the most productive and biologically diverse ecosystems, yet they are highly sensitive to human activity. The main forms of human intrusion are discussed below.

1. Direct physical damage

• Coral mining and quarrying: Corals are mined for limestone to make cement, bricks and lime, and for jewellery and curios. This destroys the reef framework itself.
• Destructive fishing: Use of dynamite (blast fishing), poison (cyanide), and bottom trawling shatters coral colonies and kills reef organisms indiscriminately.
• Anchoring and tourism: Boat anchors, divers, snorkellers and careless reef-walking break branching corals and trample reef flats. Unregulated mass tourism adds sewage and litter.
• Dredging and land reclamation: Dredging for harbours, channels and sand mining smothers corals with sediment and increases turbidity, reducing light for photosynthesis of zooxanthellae.

2. Pollution

• Sedimentation: Soil eroded from deforested coastal catchments and from construction smothers polyps and blocks sunlight.
• Nutrient loading: Sewage, agricultural fertilizers and livestock waste cause eutrophication, promoting algal overgrowth that outcompetes corals.
• Chemical pollution: Pesticides, heavy metals, oil spills and industrial effluents poison corals and reduce their resistance to disease.
• Thermal pollution: Heated effluent from power plants causes coral bleaching.

3. Global climate change (indirect intrusion)

• Coral bleaching: Rising sea-surface temperature makes corals expel their symbiotic zooxanthellae, leading to starvation and death. Mass bleaching events have increased in frequency since the 1980s.
• Ocean acidification: Absorption of excess atmospheric CO₂ lowers seawater pH and reduces carbonate saturation, slowing calcification and weakening reef skeletons.
• Sea-level rise and storms: Alters depth, light and sedimentation regimes on reefs.

4. Other intrusions

• Coastal development: Construction of resorts, roads and ports alters currents, increases runoff and physically occupies reef areas.
• Introduction of invasive species: Ballast water and aquarium trade introduce competitors and predators (e.g., lionfish) that upset reef balance.
• Freshwater diversion and mangrove destruction: Change salinity and remove natural filters that protect reefs.

Consequences: Loss of biodiversity, decline in fish stocks, reduced coastal protection from waves and erosion, and economic loss to fisheries and tourism.

Control measures: Declaration of marine protected areas, banning dynamite and cyanide fishing, regulating coastal construction and tourism, treating sewage before discharge, controlling deforestation in catchments, and global reduction of greenhouse gas emissions.

Conclusion: Human intrusion, whether through direct destruction or through pollution and climate change, is the principal cause of worldwide coral reef degradation. Sustainable coastal management and international cooperation are essential for their conservation.

Parasitic adaptations of Taenia solium (pork tapeworm)

Taenia solium is an endoparasite of the human small intestine (definitive host), with the pig as intermediate host. Being a parasite living in a nutritionally rich but hostile environment, it shows the following adaptations:

1. Adaptations for attachment (to resist peristalsis and get washed out):

• Scolex (head): Bears a rosette of 4 suckers and a rostellum armed with hooks (hence "armed tapeworm"). These grip the intestinal mucosa firmly.
• Neck: Short, undifferentiated region behind the scolex; site of continuous new proglottid formation.

2. Adaptations for protection:

• Tegument (body covering): Thick, resistant cuticle-like syncytial tegument that resists host digestive enzymes.
• No digestive system: Avoids attack by digestive juices; absorption occurs directly.
• Antibody resistance: Secretes substances that neutralize host antibodies; lives in the small intestine where immune attack is weak.

3. Adaptations for nutrition (absorption):

• Flattened, ribbon-like body: Large surface-to-volume ratio for maximum absorption of pre-digested food.
• Microtriches: Minute projections on the tegument further increase absorptive surface.
• Absorbs ready-made nutrients (glucose, amino acids) from host intestinal contents — no need for a digestive tract.

4. Adaptations for reproduction (high fecundity):

• Hermaphrodite: Each mature proglottid contains both male and female reproductive organs → self-fertilization possible.
• Gravid proglottids: Posterior segments packed with thousands of eggs; enormous egg output compensates for heavy loss during transmission.
• Gravid proglottids detach and pass out with faeces, or actively crawl out, aiding dispersal.

5. Adaptations for anaerobic respiration:

• Lives in an oxygen-poor habitat; obtains energy by anaerobic breakdown of glycogen stored in its tissues.

6. Adaptations for host entry and dispersal:

• Oncosphere (hexacanth larva): Has 6 hooks to penetrate pig intestinal wall and reach tissues.
• Cysticercus (bladder worm): Larval stage in pig muscle; survives until eaten by humans, completing the life cycle.

6. Other adaptations:

• Absence of locomotory organs and sense organs: Not needed in a protected habitat.
• Excretory and nervous systems reduced to simple longitudinal ducts and nerve cords.

Conclusion: The tapeworm is highly specialized for an endoparasitic life — firm attachment, resistant body covering, direct absorption of host nutrients, and massive reproductive output — all of which ensure its survival and successful transmission without harming the host fatally.

Structure and sexual dimorphism in Ascaris

Structure (general):

Ascaris lumbricoides is a cylindrical, bilaterally symmetrical, triploblastic, pseudocoelomate roundworm (phylum Aschelminthes/Nematoda). It is an endoparasite of the human small intestine.

• Shape and size: Elongated, spindle-shaped, tapering at both ends. Female is larger (20–35 cm long, 3–6 mm thick) than male (15–30 cm long, 2–4 mm thick).
• Body wall: Covered by a thick, resistant, non-cellular cuticle which protects against host digestive enzymes. Below it lie the epidermis (hypodermis) and a layer of longitudinal muscle (only longitudinal, no circular muscle).
• Body cavity: A pseudocoelom (false coelom) filled with pseudocoelomic fluid, which acts as a hydrostatic skeleton and distributes nutrients.
• Digestive system: Complete tube — mouth (with three lips) → pharynx → intestine → rectum → anus. No digestive glands.
• Excretory system: Two longitudinal excretory canals joined by a transverse canal, opening by an excretory pore near the anterior end (renette cells).
• Nervous system: A nerve ring around the pharynx with dorsal and ventral nerve cords.
• Reproductive system: Sexes separate (dioecious), with marked sexual dimorphism. Fertilization is internal.

Sexual dimorphism:

Thus Ascaris shows clear sexual dimorphism, the female being larger with a straight tail and separate genital opening, while the male is smaller with a curved tail, cloaca and copulatory spicules.

Social behaviour of termites

Termites are colonial insects of the order Isoptera that live in highly organised societies and are often called "white ants" (though they are not true ants). Their colony shows true social behaviour, marked by division of labour, cooperative living, and overlapping generations.

1. Caste system (division of labour). A termite colony has three main castes:

• Reproductives (king and queen): The queen is the largest individual, with a greatly enlarged abdomen. She lays thousands of eggs and, with the king, ensures the continuity of the colony. Winged reproductives (alates) leave the nest in swarming flights to start new colonies.
• Soldiers: Sterile, wingless, with large heads and strong mandibles. They defend the colony against ants and other enemies. In some species they plug the nest entrance with their heads or eject a sticky secretion.
• Workers: The most numerous and most active caste. They are sterile, wingless and blind. They build and repair the nest, collect and store food, feed the queen and young, and care for the eggs and larvae.

2. Cooperative brood care. Eggs laid by the queen are carried by workers to special chambers. The young nymphs are fed by workers with partly digested food (trophallaxis) and saliva. Since many generations live together, older individuals help rear the young — a form of altruistic behaviour.

3. Nest building. Workers construct elaborate nests of soil, saliva and faecal matter, with chambers, galleries and ventilation shafts that keep temperature and humidity fairly constant. Some tropical species build tall mounds (termitaria).

4. Nutrition and symbiosis. Termites feed on wood and cellulose. They cannot digest cellulose themselves, so flagellate protozoans (in lower termites) or bacteria (in higher termites) in the gut break it down. Workers pass food to one another and to the queen by stomodeal and proctodeal trophallaxis. Some species cultivate fungi in "fungus gardens" for food.

5. Communication. Termites communicate mainly by pheromones (chemical signals) and by tapping and vibration. Trail pheromones guide workers to food; alarm pheromones from soldiers warn the colony; queen pheromones suppress the development of extra reproductives. Antennal tapping and head banging also pass information.

6. Defence and colony integration. Soldiers sacrifice themselves for the colony. If the queen dies, replacement reproductives may develop. The colony thus behaves as a single integrated unit (superorganism), in which sterile individuals work for the survival of the reproductive pair.

Conclusion. The social life of termites is based on caste-based division of labour, cooperative brood care, trophallaxis, chemical communication and nest building — features that make the colony a highly successful and self-sustaining social organisation.

Importance of Molluscs

Molluscs (phylum Mollusca) form the second largest phylum in the animal kingdom, with about 80,000 living species. They are of great importance in the following ways:

1. As food Many molluscs are used as food. Bivalves such as mussels, oysters and clams, and gastropods such as snails, are rich in protein. Squid and octopus (cephalopods) are popular sea foods. In Nepal, freshwater snails and mussels are eaten by some communities.

2. Source of ornaments and luxury items The shells of molluscs are used to make buttons, ornaments, decorative articles and curios. Pearls obtained from pearl oysters are highly valued as gems. The "sea silk" from Pinna and purple dye from Murex are other valuable products.

3. Source of lime and manure The calcareous shells of molluscs are burnt to obtain lime, which is used in construction and in the preparation of cement. Shell powder is also used as a source of calcium in poultry and cattle feed, and as manure to enrich the soil.

4. Food for other animals Molluscs form an important link in the food chain. They serve as food for fishes, birds, frogs, snakes and other animals. In this way they help in maintaining the balance of the ecosystem.

5. Indicators of water pollution Molluscs are very sensitive to changes in water quality. The presence or absence of certain molluscs is used to indicate the level of pollution in water bodies. Some bivalves accumulate heavy metals and pesticides in their tissues and are therefore used as biological indicators of pollution.

6. Intermediate hosts of parasites Freshwater snails such as Limnaea and Bulinus act as intermediate hosts of dangerous human parasites. Limnaea transmits Fasciola hepatica (liver fluke), while Bulinus transmits Schistosoma (blood fluke). This makes their study and control important in public health.

7. Agricultural pests Snails and slugs are serious pests of crops. They feed on tender leaves, fruits and vegetables and cause heavy damage in gardens and fields. The apple snail (Pila) damages paddy seedlings.

8. Destructive borers Shipworms (Teredo) and rock-boring bivalves bore into wooden ships, wooden piles and rocks. They cause great damage to ships and harbour structures.

9. Biological and evolutionary importance Molluscs show a wide range of body forms and are excellent material for the study of comparative anatomy, adaptive radiation and evolution. The giant squid is the largest invertebrate. They are also used in laboratory experiments and in the study of the nervous system (the giant axon of squid).

10. Medicinal and other uses Some molluscs yield substances used in medicine. Shells of cuttlefish (Sepia) are used as a source of calcium and in polishing powder. Molluscan shells are also used in the preparation of traditional medicines.

Conclusion Thus molluscs are important both constructively and destructively. They serve as food, source of lime and ornaments, indicators of pollution and agents of disease, and they also damage crops and ships. Hence their study and proper management are essential.

Extra-cellular Digestion in Starfish (Asteroidea)

1. Feeding habit. Starfish is carnivorous and feeds mainly on bivalve molluscs (mussels, oysters, clams), snails, barnacles and other slow-moving animals. It is a scavenger and predator that swallows food whole or digests it outside its body.

2. Digestive system. The digestive system consists of:

• Mouth – on the oral (lower) surface, guarded by a perioral membrane and surrounded by five groups of tube feet.
• Oesophagus – short and narrow.
• Stomach – divided into two parts:
  • Cardiac stomach – large, sac-like, thin-walled, highly muscular and eversible; lies near the mouth.
  • Pyloric stomach – smaller, lies above the cardiac stomach and gives off a pair of pyloric caeca into each arm (ten in all).
• Intestine – short, followed by a rectum and anus on the aboral (upper) surface.
• Digestive glands – the pyloric caeca (tubular glands) and the rectal caeca secrete digestive enzymes.

3. Process of extra-cellular digestion. When a starfish captures a bivalve, it assumes a humped posture over the prey, attaches its tube feet to the two valves and pulls steadily until the adductor muscle of the mollusc tires and the shell gapes slightly. The starfish then everts (protrudes) its cardiac stomach through the mouth and pushes it into the shell cavity of the prey. The cardiac stomach secretes enzymes such as proteases, lipases and carbohydrases into the mantle cavity of the bivalve. Digestion of the soft body of the prey therefore takes place outside the body of the starfish, in the cavity of the prey's shell — this is true extra-cellular digestion. The partly digested, liquefied food (soup) is then drawn back into the cardiac stomach and passed into the pyloric stomach and pyloric caeca, where digestion is completed and absorption occurs. Undigested shell fragments and other hard remains are expelled through the mouth or the anus.

4. Significance.

• It enables the starfish to feed on prey much larger than its own mouth and to attack animals protected by hard shells.
• Digestion outside the body conserves energy and allows the efficient use of the prey's own shell as a "digestion chamber".
• It makes the starfish an important predator that controls populations of mussels, oysters and other bivalves in marine ecosystems, though it is a serious pest in commercial oyster and mussel beds.

Torsion and Detorsion in Gastropods

Torsion (also called twisting) is a unique developmental process in gastropods in which the visceral mass, mantle, and mantle cavity are rotated 180° around the vertical (dorso-ventral) axis of the body during the veliger larval stage. It is a characteristic feature of the class Gastropoda.

Process of torsion:

• It occurs during the veliger larva stage, before the larva settles.
• The visceral mass, along with the shell, mantle, and mantle cavity, twists through 180° relative to the head and foot.
• The rotation is through 180° in an anticlockwise direction when viewed from above (the dorsal side).
• As a result, the mantle cavity, originally posterior, comes to lie anteriorly above the head and mouth.
• The anus, nephridiopore, and gill (ctenidium) shift to the anterior position.
• The visceral nerve connectives become twisted into a figure-of-eight (8-shaped) loop.
• After torsion, the gills project forward over the head, and the auricles of the heart come to lie anterior to the ventricle (the ventricle becomes posterior).

Significance of torsion:

• It brings the mantle cavity and gills to the front, allowing clean water to enter the mantle cavity before it is polluted by the anus.
• It provides better protection to the soft body and head by allowing them to be withdrawn into the shell.
• It helps in the development of the siphon in some forms.

Detorsion: Detorsion is the secondary reversal of torsion, in which the twisted visceral mass rotates back through 180° to its original untwisted condition. It occurs in the opisthobranch gastropods (e.g., Aplysia, Doris) and in the pulmonates (lung-bearing snails).

Process of detorsion:

• The visceral mass rotates in the opposite direction through 180°.
• The mantle cavity returns to its posterior position.
• The gills and anus again lie behind the head.
• The twisted visceral nerve connectives become untwisted.
• The shell is reduced or lost in many forms.

Significance of detorsion:

• It allows the gills to be protected within the mantle cavity.
• It is an adaptation for a creeping or burrowing mode of life.
• In some forms the original ctenidia are lost and replaced by secondary gills (e.g., in some opisthobranchs).

Conclusion: Torsion is a fundamental developmental feature of gastropods that twists the visceral mass 180°, bringing the mantle cavity anteriorly. Detorsion is its secondary reversal in opisthobranchs and pulmonates, restoring the posterior position of the mantle cavity. Both processes are important adaptations for protection and respiration in gastropods.

Herdmania has no trochophore larva.

Herdmania is a sessile ascidian (sea squirt, Subphylum Urochordata). Its development passes from the fertilised egg through cleavage and gastrula directly to a tadpole larva. The trochophore is a larva of annelids and molluscs, so a trochophore of Herdmania does not exist. What the question most probably intends is the larva of Herdmania, the tadpole (ascidian) larva, described below.

Tadpole larva of Herdmania

• A free-swimming, non-feeding larva about 1-2 mm long, with an oval trunk and a long muscular tail.
• Trunk: covered by a thin tunic. At the anterior end there are three adhesive papillae used to fix the larva to a substratum. A large pharynx with gill slits (stigmata) and an endostyle is present, with a neural (brain) vesicle containing a pigmented ocellus and a statocyst above.
• Tail: contains a notochord, a hollow dorsal nerve cord and bands of tail muscles, all chordate characters; the tail is surrounded by a tail fin of tunic.
• The larva swims for a short time (a few hours to a day), then attaches by its papillae, and undergoes retrogressive metamorphosis: the tail, notochord and nerve cord are resorbed, the brain vesicle degenerates and the pharynx and gill slits enlarge, giving the sessile adult.

 adhesive papillae   brain vesicle (ocellus, statocyst)
       \              /
        [  trunk: pharynx, gill slits  ]-------- notochord --------\
                                         nerve cord (dorsal)        tail
                                         muscle bands               (tail fin)

Significance: the larva has a notochord, a dorsal tubular nerve cord and pharyngeal gill slits, which shows that Urochordata are chordates. Loss of these features in the adult is retrogressive metamorphosis.

a. Trichomonas vaginalis A flagellate protozoan (Phylum Protozoa) that lives in the human urogenital tract and causes trichomoniasis; it has four anterior flagella, an undulating membrane and an axostyle, and is transmitted mainly by sexual contact.

b. Sycon canal system A syconoid (flagellated canal) type of canal system in which the body wall is folded to form radial canals; water enters through ostia → incurrent canals → prosopyles → flagellated radial canals (lined by choanocytes) → apopyles → spongocoel → osculum.

c. Medusa of Obelia The free-swimming, umbrella-shaped sexual stage (medusa) budded from the blastostyle of the gonozooid; it bears a velum, marginal tentacles, four radial canals and gonads, and produces gametes for sexual reproduction.

d. Filariasis A vector-borne disease caused by filarial nematodes (Wuchereria bancrofti, Brugia malayi), transmitted by Culex mosquito; the worms block lymph vessels causing lymphoedema and elephantiasis of legs and scrotum.

e. Economic importance of Sitophilus oryzae The rice weevil is a serious storage pest of rice, wheat and maize; both grubs and adults bore into and feed on grains, reducing weight, quality and germination, and making the grain unfit for sale or seed.

f. Nephridia Excretory organs of annelids (e.g., earthworm) that remove nitrogenous wastes and regulate water balance; they are of three types — septal (in each segment), integumentary (on body surface) and pharyngeal (in the pharynx).

g. Mites and diseases Mites are small arachnids that act as vectors and parasites; Sarcoptes scabiei causes scabies, Demodex causes demodicosis, and trombiculid mites transmit scrub typhus (rickettsial disease) to humans.

h. Statocyst of Unio A fluid-filled sac (statocyst) located in the foot that contains calcareous statoliths; it is a balancing (equilibrium) organ that helps the animal maintain its correct posture and orientation.

i. Pedicellariae Small pincer-like appendages on the body surface of echinoderms (e.g., starfish) borne on stalks; they keep the surface clean by removing debris and larvae, and help in capturing prey and defence.

j. Coral bleaching The loss of symbiotic zooxanthellae (and their pigments) from coral tissues, making the coral white; it is caused mainly by rise in sea-water temperature, pollution and ocean acidification, and often leads to death of the coral colony.

Trichomonas vaginalis — Life cycle, pathogenicity and control measures

Systematic position Kingdom: Protista · Phylum: Sarcomastigophora · Class: Zoomastigophora · Genus: Trichomonas · Species: vaginalis

It is a flagellate protozoan that causes trichomoniasis, a common sexually transmitted infection (STI) of the human urogenital tract. It exists only as a trophozoite — no cyst stage is formed.

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1. Life cycle

Habitat: Vagina, urethra, and prostate; occasionally the urinary bladder.

Morphology of trophozoite: Pear-shaped, 7–23 µm long and 5–15 µm wide. It bears 4 anterior flagella and a 5th flagellum running backward along the surface as a undulating membrane (which ends in a free trailing flagellum). A prominent axostyle projects posteriorly. The single nucleus lies anteriorly with a small blepharoplast at its base. The cytoplasm contains siderophilic granules, vacuoles and ingested bacteria.

Mode of infection: Direct transfer of trophozoites from an infected person to a healthy person during sexual intercourse. Non-venereal transmission is rare but may occur through contaminated moist articles (towels, undergarments), toilet seats, or during childbirth from an infected mother.

Multiplication: Only binary fission (longitudinal), in the urogenital tract. The nucleus divides first, followed by the blepharoplast and other organelles.

Stages: Only trophozoite — no cyst, no spore. The trophozoite is the infective as well as the diagnostic stage. It cannot survive long outside the host (dies within minutes to a few hours in a dry environment).

Cycle in brief: Trophozoite in vagina/urethra → binary fission → large numbers in urogenital lumen → transmission to new host by sexual contact → trophozoite establishes in the new host.

Incubation period: 4–28 days (usually 7–21 days).

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2. Pathogenicity

Virulence factors:

• Adhesins (e.g. AP65, AP51, AP33) — mediate attachment to vaginal epithelial cells.
• Cysteine proteinases — degrade host immunoglobulins (IgA, IgG) and extracellular matrix, aiding invasion.
• Pore-forming proteins (trichoporeins) — lyse host cells.
• Lipophosphoglycan — damages epithelium and resists complement.
• Flagella and undulating membrane — provide motility and mechanical irritation.

Pathogenesis: The trophozoites attach to the squamous epithelium of the vagina and urethra, multiply, and cause desquamation and inflammation of the mucosa. The resulting inflammatory exudate produces the characteristic discharge. The organism does not invade deeply, so infection remains largely superficial.

Clinical features:

• In females: Vulvovaginitis with profuse, frothy, greenish-yellow, foul-smelling vaginal discharge; vulval itching and burning; dyspareunia; dysuria; strawberry cervix (punctate haemorrhages) in some cases. The vaginal pH rises above 4.5.
• In males: Often asymptomatic carriers; when symptomatic — urethritis, thin watery discharge, dysuria, prostatitis, epididymitis.
• Complications: Increased risk of HIV acquisition and transmission, preterm delivery, low birth weight, pelvic inflammatory disease, infertility.

Diagnosis:

• Microscopy of wet mount of vaginal/urethral discharge — motile trophozoites with jerky movement.
• Stained smear (Giemsa, Giemsa–Wright, Papanicolaou).
• Culture in Diamond's medium or Kupferberg's medium.
• Molecular methods (PCR) — most sensitive.
• Serology is of little value.

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3. Control measures

Treatment:

• Metronidazole (or tinidazole) — drug of choice; 2 g single oral dose or 500 mg twice daily for 7 days.
• Both sexual partners must be treated simultaneously to prevent reinfection.
• Avoid alcohol during metronidazole therapy (disulfiram-like reaction).
• In pregnancy, metronidazole is used with caution after the first trimester.

Preventive and control measures:

1. Health education about safe sexual practices and the mode of transmission.
2. Use of condoms during intercourse.
3. Screening and treatment of asymptomatic carriers, especially in high-risk groups.
4. Personal hygiene — avoid sharing towels, undergarments and other moist articles.
5. Proper disinfection of toilets, baths and clinics.
6. Case finding and contact tracing — examine and treat all contacts.
7. Avoidance of multiple sexual partners.
8. Improved living standards and sanitation.

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Conclusion: Trichomonas vaginalis is a flagellate protozoan with a simple life cycle consisting only of the trophozoite stage, transmitted mainly by sexual contact. It causes trichomoniasis with characteristic frothy vaginal discharge. Metronidazole combined with partner treatment and preventive measures effectively controls the infection.

Canal system

The canal system is the system of pores, canals and chambers running through the body wall of a sponge. Water is drawn in and driven out by the beating of flagella of the choanocytes. It does the work that other animals do with organs:

• Nutrition: water brings food particles (bacteria, plankton), which are caught by the choanocytes.
• Respiration: it brings dissolved oxygen and removes carbon dioxide.
• Excretion: it carries away wastes.
• Reproduction: it carries sperms in and larvae out.

Three types are recognised: asconoid, syconoid and leuconoid, in order of increasing complexity.

Syconoid type (e.g. Sycon, Grantia)

The body wall is folded so that the sponge has a single large central cavity, the spongocoel, which opens by one osculum. The wall is thrown into finger-like outfoldings, so the choanocytes are confined to radial canals and not to the whole spongocoel (as in asconoids).

Path of water:

1. Ostia (dermal pores) on the surface open into
2. incurrent canals (blind-ended, lined by pinacocytes), which lead through
3. prosopyles (small pores) into
4. radial canals (flagellated chambers lined with choanocytes), which open through
5. apopyles into the
6. spongocoel (lined by pinacocytes), and out through the
7. osculum.

        osculum (exit)
           |
     +-----------+
     |spongocoel |   <- lined by pinacocytes
     |  ^  ^  ^  |
     |  apopyle  |
     | radial canals (choanocytes)
     |  ^  ^  ^  |
     | prosopyles|
     | incurrent canals
     |  ^  ^  ^  |
     +--ostia----+
   water enters  ->

Features of the syconoid type: body wall thicker than in asconoids, radial canals with choanocytes, and a spongocoel that is not flagellated. The water current is stronger than in asconoids, so larger sponges are possible. It is intermediate between the asconoid and leuconoid types.

Vermicomposting is the process of converting biodegradable organic wastes (kitchen waste, crop residues, animal dung, etc.) into a nutrient-rich, humus-like material called vermicompost (also called worm compost or worm castings) through the action of earthworms, mainly Eisenia fetida (red wiggler) and Eudrilus eugeniae, along with associated microorganisms. The earthworms ingest the organic matter, and the material that passes through their gut is excreted as nutrient-rich vermicast, which forms the final product.

Preparation of Vermicompost

1. Selection of site A cool, shaded place (avoiding direct sunlight and rain) is chosen. A thatched shed or tree shade is ideal, since earthworms die above about 35–40 °C.

2. Construction of bed or pit

• Bed method: A bed of size 2 m × 1 m × 0.3 m is made on a hard floor using bricks or stones.
• Pit method: A pit of size 2 m × 1 m × 0.3 m is dug in loamy soil.
• A layer of sand (5–10 cm) is spread at the bottom for drainage, followed by a thin layer of loamy soil or cow dung to provide a base for the worms.

2. Collection and partial decomposition of waste Biodegradable wastes such as vegetable peels, leaves, grass, and cattle dung are collected. Non-biodegradable materials (plastics, glass, metals) and oily, spicy, or acidic wastes are removed. The waste is heaped and kept for 7–15 days for partial decomposition (pre-decomposition), turning it occasionally to release heat and gases that would otherwise kill the worms.

3. Introduction of earthworms The partially decomposed waste is spread in the bed in layers of about 15–20 cm. Earthworms are released at the rate of about 1 kg (≈1000 worms) per 1 m² of bed. The bed is kept moist by sprinkling water; a moisture level of 60–70 % and a temperature of 20–30 °C are maintained. The bed is covered with straw, gunny bags, or banana leaves to retain moisture and darkness.

4. Maintenance Water is sprinkled every 2–3 days (not too much, to avoid waterlogging). The bed is turned gently every 1–2 weeks without disturbing the worms. Fresh waste can be added in thin layers on top as the previous layer is consumed.

5. Harvesting In about 45–60 days, the waste is fully converted into dark, granular, odourless vermicompost. Watering is stopped for 2–3 days so that the worms move to the bottom; the compost is then collected from the top and sieved. The worms are separated and reused for the next cycle. The compost is dried in shade, packed, and stored.

Significance of Vermicompost

1. Nutrient-rich manure: It contains more N, P, K, Ca, Mg, and micronutrients (Fe, Zn, Mn, Cu) in available form than ordinary farmyard manure, and also contains humic acids, vitamins, and growth hormones (auxins, cytokinins) that promote plant growth.
2. Improves soil health: It increases soil organic matter, improves soil structure, porosity, water-holding capacity, and aeration, and prevents soil erosion.
3. Increases crop yield: Application improves seed germination, root growth, flowering, and fruiting, giving higher yield and better quality produce.
4. Waste management and sanitation: It converts biodegradable solid waste into useful manure, reducing landfill load, bad odour, and breeding of flies and mosquitoes; it is thus an eco-friendly recycling technology.
5. Reduces chemical fertilizer use: It supplies nutrients slowly and steadily, reducing dependence on costly chemical fertilizers and their harmful effects (soil degradation, water pollution).
6. Pest and disease suppression: It contains beneficial microbes and chitinase-producing organisms that suppress soil-borne pathogens and nematodes.
7. Economic benefits: It is low-cost, can be prepared at home or farm level, and provides extra income through the sale of compost and worms (vermiculture).
8. Environmental protection: It reduces greenhouse gas emission and pollution from burning or dumping organic waste, and supports sustainable organic farming.

Life history of Aedes aegypti

Aedes aegypti is a holometabolous mosquito showing complete metamorphosis. Its life cycle is completed in four stages — egg, larva, pupa and adult — and takes about 8–10 days under favourable conditions (25–30 °C).

1. Egg. The female lays 100–200 eggs singly on the inner wall of artificial containers just above the water line (tree holes, tyres, pots, tanks, discarded tins). Eggs are black, oval and cemented to the surface. They are resistant to desiccation and can survive several months in the dry state; hatching occurs when they are flooded. Aedes aegypti eggs are thus "drought-resistant", unlike those of Anopheles and Culex.

2. Larva. The egg hatches into a larva (wiggler) within 24–48 hours. The larva has a distinct head, thorax and abdomen, a siphon at the posterior end and hangs at an angle below the water surface to breathe atmospheric air. It feeds on organic matter, bacteria and algae, and moults four times (four larval instars) in about 5–7 days.

3. Pupa. The fourth-instar larva transforms into a comma-shaped pupa (tumbler). It is active but does not feed; it takes oxygen through two respiratory trumpets. The pupal stage lasts 1–2 days.

4. Adult. The pupa splits and the adult emerges on the water surface. Adults live 2–4 weeks. The female is anthropophilic (prefers human blood) and endophilic/endophagic (bites indoors, mostly during the day, with peaks in the early morning and late afternoon). Males feed on plant nectar and do not bite. Mating occurs in flight near the host; the female requires a blood meal for egg maturation. Aedes aegypti is a container breeder, is domestic and is a strong flier but usually stays within 100–200 m of its breeding site. It is the vector of dengue, chikungunya, Zika and yellow fever.

Economic importance

Harmful aspects:

1. Vector of human diseases. It transmits dengue (DEN-1 to DEN-4), chikungunya, Zika and yellow fever viruses. In Nepal, dengue is now endemic in the Terai and inner Terai, with large outbreaks in recent years; Ae. aegypti is the principal vector.
2. Public health burden. Dengue causes high morbidity, hospitalisation, absence from school and work, and occasional deaths. It imposes heavy costs on families and the State for treatment, vector control (insecticides, larvicides, fumigation) and surveillance.
3. Loss of productivity and tourism. Outbreaks reduce labour output and discourage tourism in affected areas.
4. Nuisance biting. Daytime biting disturbs rest and work.
5. Secondary economic effects. Blood loss, anaemia and allergic reactions from bites; loss of blood donors during epidemics.

Beneficial aspects:

1. Food chain. Larvae and adults serve as food for fish, frogs, dragonflies, lizards and birds.
2. Research use. Ae. aegypti is a model organism in genetics, physiology and vector biology; it is used in studies on insecticide resistance and in Wolbachia-based biological control.
3. Pollination. Males and non-blood-feeding females visit flowers and help in pollination to a minor extent.
4. Biological control agent. Toxorhynchites larvae and larvivorous fish (Gambusia) reared against Ae. aegypti form part of integrated vector management.

Control measures: elimination of stagnant water in containers (source reduction), covering water tanks, use of larvivorous fish, larvicides and adulticides, indoor residual spraying, mosquito nets and repellents, and public health education.

Pila globosa is an amphibious freshwater apple snail. It breathes air or water using two organs, so it can live both in water and out of it.

Respiratory organs

1. Ctenidium (gill): a single, left, comb-like gill (monopectinate) lying in the left part of the mantle (pallial) cavity. It is a long axis bearing a row of flat, triangular lamellae, richly supplied with blood. It is used for aquatic respiration.
2. Pulmonary sac (lung): a thin-walled, highly vascular sac on the right side of the mantle cavity, opening into it. It is used for aerial respiration. The mantle cavity also opens to the outside through a siphon, formed by a fold of the mantle margin, which is pushed out to the water surface to take in air.

Mode of respiration

• In water (aquatic respiration): the siphon takes water into the mantle cavity, where it passes over the ctenidium. Gaseous exchange occurs across the gill lamellae, and the water leaves through the siphon. The osphradium, a sensory organ at the base of the gill, tests the water.
• On land or in stagnant, oxygen-poor water (aerial respiration): the snail rises to the surface and extends the siphon above the water. Air passes to the pulmonary sac, where oxygen diffuses into the blood. During dry weather, Pila closes its shell with the operculum and aestivates in mud, breathing very slowly through the pulmonary sac.

Because of the two organs, Pila is called amphibious. The branchial and pulmonary respiration may alternate in the same animal.

Amphioxus (Branchiostoma, the lancelet) is a small, fish-like marine cephalochordate. It combines chordate, invertebrate and vertebrate features, so it is important for understanding the origin of vertebrates.

1. Chordate characters

• A notochord along the whole body, persisting throughout life.
• A dorsal, hollow nerve cord.
• Pharyngeal gill slits (numerous).
• A post-anal tail.
• An endostyle on the pharynx floor (homologous with the thyroid of vertebrates).

2. Affinities with invertebrates

• Annelids: metameric body, with segmental myotomes, gonads and excretory organs. The excretory organs are protonephridia (solenocytes), as in some annelids.
• General invertebrate features: no distinct head or brain, no paired appendages, no vertebral column, no true heart, a ciliary-mucous feeding method, and a simple epidermis of one cell layer.
• Echinoderms and hemichordates: the embryology (radial cleavage, enterocoelic coelom, deuterostome mouth) is shared with them, and the early larva of Branchiostoma is ciliated and free-swimming like their larvae.

3. Affinities with vertebrates

• Notochord, dorsal nerve cord and pharyngeal clefts, as seen in a vertebrate embryo.
• A closed circulatory system with a ventral aorta and a hepatic portal system, with a hepatic caecum corresponding to the liver.
• Myotomes of V-shape (chevron) and a dorsal fin.
• The endostyle compares with the thyroid.

4. Primitive features not seen in vertebrates: no skull, brain, vertebrae or paired fins, and no true heart (contractile vessels), and no kidneys.

Conclusion: Branchiostoma is a primitive chordate that links invertebrates and vertebrates, and is the closest living model of the ancestral chordate.

Morphometric Methods in Taxonomy [5 marks]

Introduction. Morphometrics is the quantitative study of shape and size variation in organisms. In taxonomy it provides objective, measurable and repeatable characters that supplement qualitative descriptions, helping to delimit species, detect cryptic taxa and infer relationships.

Types of morphometric methods

1. Traditional (univariate and bivariate) morphometrics. Individual distances and ratios are measured with a scale, calipers or ocular micrometer — e.g. body length, body width, head length, wing span, internode length. Data are summarised as mean, standard deviation and coefficient of variation, and compared between populations by t-test or analysis of variance (ANOVA). Bivariate plots (e.g. length against width) and simple ratios (e.g. head length/body length) remove the effect of overall size.

2. Multivariate morphometrics. Many correlated characters are measured on each specimen and analysed together, so that the total pattern of variation is captured:

   • Principal Component Analysis (PCA) — reduces many correlated variables to a few uncorrelated components; PC1 usually expresses overall size, PC2 and later components express shape.
   • Discriminant Function Analysis (DFA) — finds the combination of characters that best separates previously defined groups, and is used to assign unknown specimens to species or populations.
   • Cluster analysis and canonical variate analysis — group specimens by overall similarity and display relationships as phenograms or scatter plots.
   • Multivariate analysis of variance (MANOVA) — tests whether group means differ significantly for the whole set of characters.

3. Geometric morphometrics. Landmarks (biologically homologous points) and outlines are digitised and analysed as coordinates, so that shape is studied independently of size, position and orientation:

   • Procrustes superimposition (generalised least squares) aligns configurations by translation, rotation and scaling.
   • Thin-plate spline visualises shape change as deformation grids and gives partial warp scores for further analysis.
   • Relative warp and principal component analysis of Procrustes coordinates summarise major shape trends.
   • Elliptic Fourier and eigenshape analysis handle outlines and curves where fixed landmarks are scarce (e.g. leaves, shells, otoliths).

Applications in taxonomy.

• Discriminating closely related and sibling species where qualitative characters overlap.
• Detecting and describing cryptic species and geographic variation (clines, subspecies).
• Assessing sexual dimorphism and ontogenetic (growth) changes that may be mistaken for species differences.
• Testing the distinctness of fossil taxa and reconstructing evolutionary shape change.
• Providing numerical characters for keys, descriptions and phylogenetic analysis.

Requirements and limitations. Large, representative samples of known sex and age are needed; landmarks must be truly homologous; measurement error must be controlled; and results are only as good as the sampling. Morphometric data alone cannot establish reproductive isolation, so they must be combined with genetic, ecological and behavioural evidence.

Conclusion. Morphometrics converts morphology into objective numerical data. Traditional methods are simple and widely used, while multivariate and geometric methods give a far more powerful and visual picture of shape variation, making them indispensable tools in modern taxonomy.

Vorticella is a stalked, bell-shaped peritrich ciliate. It multiplies asexually by binary fission and budding, and sexually by conjugation.

Conjugation in Vorticella

It is anisogamous (heteromorphic), because the two conjugants are unequal:

• The macroconjugant is a normal, large, sessile, stalked individual.
• The microconjugant is a small, free-swimming, stalkless individual that arises by unequal fission of an ordinary individual. It is ciliated at its lower end and swims away to find a macroconjugant.

Stages

1. The microconjugant attaches to the macroconjugant, and the two cytoplasms become continuous at the point of contact.
2. The macronucleus of each partner degenerates. The micronucleus divides meiotically, and most of the resulting nuclei degenerate, leaving a stationary and a migratory pronucleus.
3. The migratory pronucleus of the microconjugant moves into the macroconjugant and fuses with its stationary pronucleus to form the synkaryon (zygote nucleus). The microconjugant itself is absorbed or dies, and does not survive.
4. The synkaryon divides repeatedly. Some of the products form the new micronucleus and macronucleus, and the macroconjugant returns to normal life, with renewed vigour.

Significance: it causes nuclear reorganisation and genetic recombination. In contrast to Paramecium, where the partners separate and both survive, in Vorticella only the macroconjugant survives.

Corals & Coral Reefs

Corals are marine animals belonging to the phylum Cnidaria (class Anthozoa). They exist as small, cylindrical polyps that live in colonies. Each polyp has a sac-like body with a mouth surrounded by tentacles and a calcareous (calcium carbonate) skeleton secreted at its base. Most reef-building corals live in a symbiotic relationship with zooxanthellae, single-celled dinoflagellate algae that live within their tissues. The algae carry out photosynthesis and supply the coral with organic food and oxygen, while the coral provides the algae with shelter and nutrients. Corals also capture plankton using their stinging cells (nematocysts). They reproduce both asexually (budding, fragmentation) and sexually (by releasing gametes into water).

Coral reefs are massive, wave-resistant, calcareous structures built up over thousands of years by the accumulation of coral skeletons and the calcareous remains of other organisms such as molluscs and coralline algae. They are the most diverse marine ecosystems, often called the "rainforests of the sea."

Conditions required for reef formation:

• Warm water (20–30 °C)
• Shallow, clear water (generally less than 50 m deep) so sunlight reaches the zooxanthellae
• Saline water (about 27–40 ppt)
• Low turbidity and strong wave action that supplies oxygen and food

Types of coral reefs (Darwin's classification):

1. Fringing reef — grows close to the shore, directly attached to land (e.g., Red Sea coasts).
2. Barrier reef — lies offshore, separated from land by a lagoon (e.g., Great Barrier Reef, Australia).
3. Atoll — a ring-shaped reef enclosing a central lagoon, formed on subsiding volcanic islands (e.g., Maldives, Lakshadweep).

Importance:

• Act as natural barriers, protecting coastlines from erosion and storm waves.
• Provide habitat and breeding grounds for a huge diversity of fish and invertebrates.
• Support fisheries and tourism, contributing to the economy.
• Used as a source of lime, building material and ornamental products.

Threats: Coral bleaching due to global warming, ocean acidification, pollution, sedimentation, destructive fishing and over-exploitation.

Conclusion: Corals are tiny colonial cnidarians that build coral reefs — biologically rich, economically valuable marine ecosystems that need strict conservation for their survival.

Amoebiasis (Amoebic Dysentery)

Amoebiasis is a protozoan disease of the human large intestine caused by Entamoeba histolytica. The infective stage is the mature quadrinucleate cyst, which is transmitted through contaminated food and water (faecal–oral route). The trophozoite is the invasive, disease-producing stage.

Symptoms

1. Intestinal (amoebic dysentery): The incubation period is usually 2–4 weeks. Symptoms include:

   • Frequent, loose stools with blood and mucus (characteristic "anchovy sauce" stool)
   • Abdominal pain and cramps, especially in the lower right side
   • Tenesmus (painful urge to defecate) and flatulence
   • Diarrhoea alternating with constipation in mild cases
   • Fever, fatigue, and loss of appetite

2. Extra-intestinal (invasive) forms:

   • Amoebic liver abscess — fever with rigors, pain and tenderness in the right upper abdomen, enlarged tender liver, jaundice
   • Amoebic hepatitis — enlarged liver without pus formation
   • Rarely, spread to lungs (cough, chest pain), brain, or skin, causing abscesses

3. Complications: Perforation of the intestinal wall, peritonitis, toxic megacolon, and severe anaemia.

Control Measures

1. Treatment of patients and carriers: Use of anti-amoebic drugs such as metronidazole (for invasive disease) followed by luminal amoebicides (e.g., diloxanide furoate) to eliminate cysts. Treating carriers prevents spread.

2. Sanitation:

   • Proper disposal of human faeces; use of sanitary latrines
   • Protection of drinking water sources from sewage contamination
   • Safe drinking water — boiling or chlorination

3. Food hygiene: Wash vegetables and fruits in clean water; cover food to protect it from flies, cockroaches, and dust; avoid eating exposed or stale food.

4. Personal hygiene: Wash hands with soap after defecation and before eating; keep fingernails short and clean; avoid using human excreta as fertilizer.

5. Health education: Educate the public about the mode of transmission, the importance of clean water and food, and the need for early treatment.

6. Fly and vector control: Control flies and cockroaches that mechanically carry cysts, by spraying insecticides and keeping surroundings clean.

7. Screening and surveillance: Screen food handlers and water sources periodically, especially in endemic areas.

Copulation and Cocoon Formation in Leech (Hirudinaria granulosa)

Copulation (Mating)

1. Season and pairing: Leeches are hermaphrodite (bisexual) but cross-fertilizing. Copulation occurs in the rainy season (July–August) on moist soil or in shallow water, when the clitellum becomes prominent and the gonads mature.

2. Position of pairing: Two leeches attach to each other in opposite directions — the clitellum of one lies against the clitellum (or the gonopore region) of the other. The anterior sucker of each grips the body of the partner.

3. Transfer of sperm: Each leech is both donor and receiver. The male gonopore (on the 10th segment) of one is applied to the female gonopore (on the 11th segment) of the other. The penis, or a spermatophore, transfers spermatozoa into the vagina of the partner. Thus sperm is exchanged mutually (reciprocal insemination).

4. Storage of sperm: The received spermatozoa are stored in the spermathecae (two pairs, in segments 9 and 10), where they remain alive until fertilization.

Cocoon Formation

1. Role of clitellum: After copulation, the clitellum (segments 9–11) becomes swollen and glandular. Its epidermal cells secrete a mucilaginous, albuminous fluid that hardens on contact with air/water.

2. Slipping of the cocoon: By peristaltic movements of the body, the leech withdraws backwards out of this ring of secretion. The ring therefore slips forward over the anterior segments like a sleeve.

3. Eggs and sperm: As the ring passes over the female gonopore (11th segment), ova (about 10–15 eggs) are deposited into it. As it passes over the spermathecae (9th and 10th segments), the stored spermatozoa are released into it, so fertilization occurs inside the cocoon.

4. Albuminous coat: As the ring passes over the male gonopore and the anterior region, more albuminous secretion is added, forming a nutritive albuminous layer around the eggs.

5. Hardening: The ring finally slips off the anterior end (over the mouth) and its ends close, forming a lemon-shaped, brownish, horny cocoon — the "egg case".

6. Deposition and development: The cocoon is deposited in moist soil or mud near water. Inside it, eggs undergo direct development (no larval stage); young leeches hatch out after about 2–4 weeks as miniature adults.

Key points to remember: Hermaphrodite but cross-fertilization → reciprocal sperm transfer → spermathecae store sperm → clitellum secretes cocoon → eggs + sperm enter cocoon → fertilization inside → cocoon hardens and is laid in moist soil → direct development.

Metamorphosis is the series of marked structural, physiological and behavioural changes by which an immature (larval) insect becomes an adult. It is controlled by hormones: ecdysone from the prothoracic glands brings about moulting, and juvenile hormone from the corpora allata decides whether the next stage is larval, pupal or adult.

Types of metamorphosis in insects

1. Ametabolous (no metamorphosis): the young (juvenile) resemble the adult except for size and sexual maturity, and grow through moults. Examples: silverfish (Lepisma), springtails.
2. Hemimetabolous (incomplete metamorphosis): stages are egg, nymph and adult. The nymph resembles the adult but has only wing pads, which become fully developed wings after several moults. In dragonflies, mayflies and stoneflies the aquatic young are called naiads. Examples: cockroach, grasshopper, bug, dragonfly.
3. Holometabolous (complete metamorphosis): four stages, egg, larva, pupa and adult (imago). The larva (caterpillar, grub or maggot) is very different from the adult. In the non-feeding pupa the larval tissues are broken down (histolysis) and adult organs are built from imaginal discs (histogenesis). Examples: butterfly, silk moth, housefly, mosquito, beetle, honeybee.

Significance: larva and adult use different food and habitats, which reduces competition within the species.

Torsion

Torsion is a twisting of the visceral mass, relative to the head and foot, through 180 degrees in an anticlockwise direction (as seen from above). It happens in the veliger larva of gastropod molluscs and is a unique feature of the class. It is brought about mainly by differential growth, and occurs in a few hours or a short period in different species.

Effects of torsion:

• The mantle cavity, with the anus, gills and kidney openings, comes to lie in front, over the head.
• The nervous system is twisted into a figure of eight (streptoneury, a crossed visceral loop).
• Organs on one side (typically the right gill, auricle and kidney) are lost or reduced, giving an asymmetrical adult.
• The head can be withdrawn into the mantle cavity for protection.
• Disadvantage: the anus and excreta now lie over the head, which is a risk for fouling.

Detorsion

Detorsion is the partial or complete reversal of torsion that happens in some advanced gastropods (e.g. opisthobranchs such as sea hares and nudibranchs, and pulmonates such as slugs). The visceral mass untwists, the anus and mantle cavity move to the right side or posterior, and the nerve loop may be uncrossed (euthyneury). Many show reduction or loss of the shell and the mantle cavity.

Summary: torsion is a larval twist of $180^\circ$ that gives the typical gastropod asymmetry, while detorsion is a secondary untwisting of this.

Tube Feet of Star Fish

Tube feet (podia) are the locomotory and respiratory organs of the water-vascular system of Asterias (star fish). They are small, hollow, muscular, sucker-like projections that arise in rows from the radial canals and project out through the ambulacral grooves on the oral surface of each arm.

Structure

• Each tube foot is a thin-walled, extensible tube made of ciliated epithelium enclosing a fluid-filled cavity that is continuous with the ampulla.
• Its base carries a bulb-like ampulla lying inside the arm, and its free end bears a terminal sucker or disc.
• The wall contains longitudinal muscle fibres and connective tissue; the ampulla has its own muscle fibres.
• The terminal disc is glandular and adhesive, and is provided with a small calcareous plate (terminal plate) for support.

Arrangement

• Tube feet occur in four rows in each ambulacral groove (two on each side of the radial canal), so that a star fish may possess about 2,000 tube feet in all.
• Each row is supplied by short lateral branches (podial canals) given off from the radial canal.

Working (Locomotion)

1. Water enters the tube foot from the radial canal through the podial canal.
2. Contraction of the ampulla forces water into the tube foot, which therefore elongates and extends forward.
3. The sucker is pressed against the substratum and fixed by suction and by the secretion of the terminal glands.
4. The longitudinal muscles of the tube foot now contract, shortening it and pulling the body forward.
5. Water is driven back into the ampulla, the sucker is released, and the process is repeated.

Functions

• Locomotion: by the coordinated stepping action of thousands of tube feet, aided by the suckers.
• Respiration: the thin, ciliated, moist wall serves as a gill for the exchange of gases with surrounding water.
• Food capture and burrowing: tube feet help in opening bivalve shells, in capturing prey, and in gripping and burrowing into sand or mud.
• Attachment: they anchor the animal firmly to rocks and other objects.

Thus the tube feet form a hydraulic sucking mechanism worked by the water-vascular system, and are the chief organs of locomotion, respiration and attachment in the star fish.

Life cycle of Ascaris lumbricoides

Ascaris lumbricoides, the large roundworm, is an endoparasite in the small intestine of man. It is monogenetic: its life cycle is direct and completed in a single host (man), with no intermediate host.

1. Mode of infection Man is infected by swallowing embryonated eggs (containing the infective larva) through contaminated food, water, raw vegetables or soiled hands (faecal-oral route).

2. Hatching The egg-shell is digested in the small intestine and the larva hatches. It bores through the intestinal wall and enters the blood and the hepatic portal system.

3. Migration (tracheal migration) The larvae pass by blood to the liver, right heart and lungs. In the lungs they grow and moult, break through the alveolar wall, and climb up the bronchi and trachea. They are coughed up to the pharynx and swallowed into the oesophagus and stomach.

4. Maturation In the small intestine the larvae undergo the last moult and become sexually mature adults in about 2 to 3 months.

5. Eggs Adults mate, and a female lays about $2\times10^5$ eggs a day. The fertilised eggs pass out in faeces. In warm, moist soil, an egg develops into an infective embryonated egg in about 2 to 3 weeks. It stays viable for months.

Summary: infective egg $\rightarrow$ intestine $\rightarrow$ liver $\rightarrow$ heart $\rightarrow$ lungs $\rightarrow$ trachea $\rightarrow$ swallowed $\rightarrow$ intestine (adult) $\rightarrow$ eggs in faeces $\rightarrow$ soil $\rightarrow$ infective egg.

Effects: heavy infection causes ascariasis, with abdominal pain, malnutrition (especially in children) and sometimes intestinal obstruction.

Silk of Moth and Its Uses

Silk is a natural protein fibre produced by the larvae (caterpillars) of certain moths, the most important being the silkworm, Bombyx mori (family Bombycidae). Silk is secreted by the caterpillar as a continuous filament to form its protective covering, the cocoon, within which it pupates.

Source and formation:

• The silk glands of the caterpillar secrete a viscous fluid through a pair of openings called spinnerets located on the lower lip (labium).
• The fluid is a protein called fibroin, cemented together by a gum-like protein called sericin.
• On exposure to air, the fluid hardens into a fine, continuous thread. The caterpillar winds this thread around itself, forming the cocoon in about 2–3 days.
• A single cocoon may contain a filament 600–900 metres long.

Types of silk:

1. Mulberry silk — from Bombyx mori, which feeds on mulberry leaves; the finest and most commercial silk.
2. Non-mulberry (wild) silk — e.g. Tasar silk from Antheraea mylitta (feeds on oak/terminalia), Eri silk from Attacus/Samia ricini (castor leaves), and Muga silk from Antheraea assamensis (Assam, golden-yellow silk).

Processing (brief): Cocoons are harvested and stifled (killed by heat/steam) to preserve the filament. They are then reeled (unwound) in hot water, which softens the sericin so the filaments separate. Several filaments are thrown/twisted together to make yarn, which is degummed (sericin removed), dyed, and woven into cloth. The pupae left after reeling are used as food or feed.

Uses of silk:

1. Textiles — sarees, shawls, scarves, ties, dresses, and other luxury garments due to its lustre, smoothness, and lightness.
2. Sewing and embroidery — silk thread is used for stitching and decorative embroidery.
3. Medical uses — silk sutures (biocompatible, strong, and non-absorbable, so they are removed or left as permanent stitches) for wound stitching; silk is also used in tissue engineering and as a dressing material.
4. Parachutes and cords — silk fabric was historically used for parachutes, balloon cloth, and strong cords.
5. Fishing — silk gut/thread used in angling lines.
6. Cosmetics and pharmaceuticals — hydrolysed silk protein (sericin) is used in creams, soaps, and hair products.
7. Insulation and special uses — silk is used in insulating materials and in some electronic/optical applications.
8. Economic value — sericulture (silk farming) is a major agro-based industry providing employment and income, especially in China, India, and Nepal.

Conclusion: Silk, secreted by the silk moth larva as fibroin cemented with sericin, is a valuable natural fibre with wide applications ranging from luxury clothing and embroidery to surgical sutures and cosmetics, making sericulture an important industry.

a. Nephridia
Nephridia are segmentally arranged, ciliated, tubular excretory organs of annelids (e.g., earthworm). They remove nitrogenous wastes from coelomic fluid and discharge them to the exterior through nephridiopores.

b. Calcareous spicule and significance
Calcareous spicules are minute, needle-like skeletal elements of calcium carbonate secreted by sclerocytes in sponges. They provide support and rigidity, keep the canal system open, and also serve as a defence against predators.

c. Dactylozooids in coelenterate
Dactylozooids are defensive and food-catching zooids of polymorphic colonial coelenterates (e.g., Physalia, Hydractinia). They are mouthless, with one long tentacle bearing batteries of nematocysts that paralyse prey and protect the colony.

d. Sexual dimorphism in Ascaris
The sexes are separate and morphologically distinct. The female is longer and stouter (20–40 cm) with a straight, posteriorly pointed tail and a separate vulvar opening; the male is shorter and thinner (15–30 cm) with a curved tail bearing a cloaca and two penial setae.

e. Parasitism
Parasitism is a mode of heterotrophic nutrition in which one organism (the parasite) lives on or inside another organism (the host), obtains its food from the host's body, and harms it, while the host is deprived of nutrition and may be diseased.

f. Jaws of leech
Leech jaws are three small, semicircular, muscular and toothed ridges present in the buccal cavity, arranged in a triradiate (Y-shaped) manner. They bear many minute teeth and are used to make a triradiate bite on the host's skin during blood sucking.

g. Statocysts of prawn
Statocysts are paired balancing (equilibrium) sense organs located in the basal segments of the antennules. Each is a sac lined with sensory setae and containing statoliths (sand grains); they help the prawn maintain its normal posture and orientation in water.

h. Dentalium
Dentalium is a marine mollusc of the class Scaphopoda, commonly called the tusk shell. It has a tubular, tusk-shaped shell open at both ends, a burrowing habit, and a foot used for digging into sandy mud.

i. Retrogressive metamorphosis
Retrogressive metamorphosis is a type of development in which a free-swimming, well-developed larva (e.g., the ascidian tadpole larva with notochord and nerve cord) loses its advanced chordate characters and changes into a simpler, sessile adult, thus showing degeneration of organs.

j. Kala-azar
Kala-azar (visceral leishmaniasis, "black fever") is a disease caused by the protozoan Leishmania donovani, transmitted by the bite of the sandfly (Phlebotomus argentipes). It causes prolonged fever, enlarged spleen and liver, anaemia and darkening of the skin. It is endemic in the Terai of Nepal.

Protozoa

Protozoa (Greek: protos = first, zoon = animal) are unicellular, eukaryotic, microscopic animals. They are holozoic or parasitic, lack chlorophyll and a cell wall, and perform all life activities within a single cell. Locomotion is by pseudopodia, flagella or cilia. They reproduce asexually (binary fission, multiple fission) and in some cases sexually (syngamy, conjugation). Examples: Amoeba, Paramecium, Trypanosoma, Leishmania, Plasmodium.

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Leishmania donovani

Systematic position

It is a haemoflagellate causing visceral leishmaniasis (kala-azar), also called Dum-Dum fever or black sickness.

Morphology

Two forms occur:

1. Amastigote (LD body) — found in man. Round/oval, 2–4 µm, with a nucleus, a kinetoplast and a rudimentary flagellum (no free flagellum). It is an obligate intracellular parasite of reticuloendothelial cells.
2. Promastigote (leptomonad) — found in the sandfly vector and in culture. Slender, 15–20 µm, with a nucleus, kinetoplast and a free anterior flagellum arising from a blepharoplast.

Life cycle (digentic / two-host)

Hosts:

• Definitive host: man (also dog, jackal — reservoir)
• Intermediate host / vector: female sandfly (Phlebotomus argentipes)

Steps:

1. In man (asexual cycle): The infected sandfly bites and injects promastigotes into the skin. They enter macrophages and other reticuloendothelial cells and transform into amastigotes. These multiply by binary fission until the cell ruptures, releasing amastigotes that infect fresh macrophages. They spread via blood and lymph to the spleen, liver, bone marrow and lymph nodes, producing the disease.

2. In sandfly (sexual/flagellate cycle): When the sandfly bites an infected person, it sucks up amastigotes along with blood. In the midgut the amastigotes change into promastigotes, which multiply by longitudinal binary fission and migrate to the pharynx and buccal cavity. The sandfly is now infective and transmits the parasite at its next bite.

Thus the cycle is completed: man → sandfly → man. The infective stage for man is the promastigote; the diagnostic stage in man is the amastigote.

Pathogenicity (kala-azar)

• Incubation period: 2–4 months (range 10 days to 2 years).
• The parasite destroys reticuloendothelial cells, causing:
  • Prolonged irregular fever with double rise in 24 hours.
  • Splenomegaly and hepatomegaly — spleen becomes enormously enlarged and firm.
  • Anaemia, leukopenia and thrombocytopenia due to bone-marrow involvement.
  • Darkening of skin (hence "kala-azar" = black sickness), especially on face and hands.
  • Loss of weight and weakness; lymphadenopathy.
• If untreated, death occurs in about 2 years, usually from secondary bacterial infection, dysentery or haemorrhage.
• Post-kala-azar dermal leishmaniasis (PKDL): after recovery, painless skin nodules and depigmented patches appear, which act as a reservoir of infection.

Control measures

1. Treatment of patients (to cut the reservoir):

• Pentavalent antimonials — sodium stibogluconate (Pentostam), meglumine antimoniate.
• Amphotericin B (liposomal), miltefosine, paromomycin.
• Complete the full course to prevent PKDL and relapse.

2. Vector (sandfly) control:

• Residual insecticide spraying (DDT, malathion) in houses and cattle sheds.
• Use of insect repellents, nets and screening of windows.
• Filling cracks and plastering walls to destroy breeding/resting sites.
• Keeping cattle sheds away from living rooms (sandflies breed in moist organic matter).

3. Protection of man:

• Avoid sleeping on the floor, especially in endemic areas.
• Use mosquito nets (fine mesh) and insecticide-treated bed nets.
• Personal hygiene and cleanliness around the house.

4. Reservoir control:

• Detection and treatment of canine and human cases; stray dogs should be controlled in endemic zones.

5. Public health measures:

• Early diagnosis by LD body demonstration in spleen/bone-marrow aspirate, culture, or rK39 rapid test.
• Health education about the mode of transmission.
• Active case finding and surveillance in endemic districts (e.g. Bihar, eastern Nepal Terai).

Metazoa

Metazoa (Greek meta = later, zoa = animals) is the subkingdom of all multicellular, heterotrophic, eukaryotic animals, whose cells lack cell walls. It is divided into Parazoa (sponges, with cells not organised into true tissues) and Eumetazoa (all other animals, with true tissues). Scypha (formerly Sycon) is a marine calcareous sponge of class Calcarea, phylum Porifera, so it is a parazoan metazoan.

Reproduction in Scypha

Scypha reproduces both asexually and sexually.

A. Asexual reproduction

1. Budding. A bud develops on the body wall and either separates to form a new sponge or remains attached to form a colony.
2. Fragmentation. Broken pieces regenerate into complete sponges because of high regenerative power.
3. Gemmules (resistant masses of archaeocytes surrounded by spongin and spicules) are typical of freshwater sponges. Scypha is marine and does not form them.

B. Sexual reproduction

Scypha is monoecious (hermaphrodite), but eggs and sperms mature at different times (protogyny), so self-fertilisation is avoided.

• Gametes. Sperms and eggs arise from archaeocytes (and choanocytes) in the mesohyl.
• Fertilisation. Sperms leave through the osculum, enter another sponge through the ostia with the water current, and reach the eggs in the mesohyl. Fertilisation is therefore internal, with cross-fertilisation.

Embryogeny of Scypha

1. Cleavage. The zygote undergoes holoblastic, unequal cleavage, forming a solid ball of cells, the stereoblastula, with small micromeres and large macromeres.
2. Amphiblastula larva. The stereoblastula turns into a hollow amphiblastula. The flagellated micromeres lie at one pole (anterior) and the large non-flagellated granular macromeres at the other (posterior). The larva leaves through the osculum and swims freely.
3. Settlement and metamorphosis. The larva settles on its anterior end. The micromeres move inward (inversion of layers) and become choanocytes, and the macromeres spread outside to form the pinacoderm, mesohyl and archaeocytes.
4. Olynthus. The young sponge has a central spongocoel, an osculum and ostia. Its wall then folds to form the radial canals and flagellated chambers of the adult syconoid canal system.

Thus Scypha shows internal fertilisation, development through the amphiblastula larva, and metamorphosis into an adult syconoid sponge.

Wuchereria bancrofti is a filarial nematode that causes lymphatic filariasis (elephantiasis) in humans. It is transmitted by mosquitoes and is a major public health problem in the tropical and subtropical regions of Nepal, India, Africa and South America.

1. Life Cycle

W. bancrofti is a digenetic (two-host) parasite — the definitive host is man and the intermediate host is a female mosquito (Culex quinquefasciatus, Anopheles or Aedes depending on region).

a) In the human host (definitive host)

1. Entry: An infected female mosquito bites a person and injects third-stage infective larvae (L₃) into the skin.
2. Development: The larvae enter the lymphatic vessels and lymph nodes, where they develop into adult worms in about 6–12 months.
3. Adults: The adults are long, thread-like, whitish worms — female about 8–10 cm and male about 4 cm. They live coiled in the lymphatic vessels and lymph nodes, mainly of the lower limbs, groin, scrotum and pelvic region. The female is viviparous.
4. Microfilariae: The gravid female liberates microfilariae (embryos, ~250–300 µm long) into the lymph, which then enter the blood stream.
5. Periodicity: Microfilariae show a marked nocturnal periodicity — they appear in the peripheral blood mainly between 10 p.m. and 2 a.m., coinciding with the biting habit of the vector mosquito. During the day they retreat into the deep vessels of the lungs.

b) In the mosquito host (intermediate host)

5. The mosquito sucks blood containing microfilariae during its night bite.
6. In the mosquito's stomach the microfilariae lose their sheath, penetrate the stomach wall and reach the thoracic muscles.
7. There they pass through two moults: L₁ → L₂ → L₃ (sausage stage → infective stage) in about 10–14 days.
8. The infective L₃ larvae migrate to the proboscis and are ready to be injected into a new human host on the next bite, completing the cycle.

Summary of the cycle: Infected mosquito → L₃ into man → adult worms in lymphatics → microfilariae in blood (night) → mosquito takes them up → development in mosquito → infective L₃ → man.

2. Pathogenicity

The disease produced is lymphatic filariasis (filariasis / elephantiasis). Pathogenicity is due mainly to the adult worms and the host's allergic-inflammatory response to them, not to the microfilariae.

a) Incubation period: Usually 8–12 months, sometimes up to 1–2 years.

b) Clinical stages

1. Asymptomatic stage: Many persons carry microfilariae in blood without symptoms.
2. Acute stage (filarial fever):
   • Recurrent attacks of fever with lymphangitis and lymphadenitis (inflamed, painful, cord-like lymph vessels and enlarged lymph nodes, especially inguinal).
   • Filarial funiculitis and epididymo-orchitis in males.
   • Attacks last a few days to weeks and recur.
3. Chronic stage (obstructive stage):
   • Elephantiasis — gross swelling and thickening of the limb (usually leg) due to lymphatic obstruction and fibrosis; the skin becomes rough, hard and warty.
   • Hydrocele — swelling of the scrotum, very common in males.
   • Chyluria — milky urine due to rupture of lymphatics into the urinary tract.
   • Lymph scrotum / vulvar swelling in females.
   • Tropical pulmonary eosinophilia (TPE) — due to hypersensitivity to microfilariae, with cough, wheezing, eosinophilia and lung infiltrates.

c) Mechanism: Adult worms block lymph flow → lymph stasis and oedema → repeated secondary bacterial infection → fibrosis and permanent thickening (elephantiasis).

3. Control Measures

a) Vector (mosquito) control

• Elimination of breeding sites — drainage of stagnant water, proper disposal of sewage.
• Use of larvicides and adulticides; indoor residual spraying.
• Use of mosquito nets (especially insecticide-treated bed nets) and repellents.
• Screening of houses and personal protection against night biting.

b) Control of infection in man

• Mass drug administration (MDA) with diethylcarbamazine (DEC) alone or in combination with albendazole (and ivermectin in some regions) — the mainstay of the WHO Global Programme to Eliminate Lymphatic Filariasis.
• Treatment of individual cases with DEC to kill microfilariae and adult worms.
• Doxycycline to kill the Wolbachia endosymbiont of the worm, which reduces adult worm survival.
• Surgical management of hydrocele and reconstructive surgery for elephantiasis.

c) Prevention of transmission

• Avoid mosquito bites at night.
• Screening of blood is not required (no direct blood transmission), but infected persons should be treated.
• Health education about the mode of transmission, use of nets and importance of completing MDA.

d) Environmental and public health measures

• Improved sanitation and water supply.
• Regular surveillance, mapping of endemic areas and monitoring of microfilaraemia rates.
• Integrated vector management (IVM) combining chemical, biological and environmental methods.

Conclusion: W. bancrofti completes its life cycle between man and mosquito, with nocturnal periodicity of microfilariae. Its pathogenicity results from lymphatic obstruction by adult worms, producing acute filarial fever and chronic elephantiasis, hydrocele and chyluria. Control depends on mass DEC/albendazole chemotherapy, mosquito control and health education, as adopted in Nepal's national filariasis elimination programme.

Life history and economic importance of Sitophilus oryzae (Rice Weevil)

Systematic position

• Phylum: Arthropoda
• Class: Insecta
• Order: Coleoptera
• Family: Curculionidae
• Genus: Sitophilus
• Species: oryzae

Habitat and distribution Sitophilus oryzae, the rice weevil, is a cosmopolitan stored-grain pest. It infests rice, wheat, maize, barley, sorghum and their products in godowns, warehouses, mills and granaries. It is a primary pest that attacks whole, sound grains.

External features The adult is a small, dark reddish-brown to almost black beetle, about 2.5–3.5 mm long. It has a characteristic long, curved snout (rostrum) bearing the mouthparts at its tip. The body is oval and hard (chitinous), with four reddish-yellow spots on the elytra (wing covers). The elytra are hard and the membranous hind wings are folded beneath them; the insect can fly. Antennae are elbowed and clubbed. Legs are adapted for walking and clinging to grains. The larva is a small, white, legless, C-shaped grub with a soft body and a distinct head.

Life history

1. Mating and egg laying: After emergence, adults mate. The female bores a small hole in the grain with her snout, deposits a single egg inside the endosperm, and seals the hole with a gelatinous secretion. One female lays about 300–400 eggs in her lifetime (about 4 eggs per day).
2. Incubation: The egg hatches in about 3–6 days in warm weather (optimum 27–30 °C).
3. Larval stage: The legless larva feeds on the endosperm and inner parts of the grain, hollowing it out completely. It passes through four larval instars in about 18–25 days. Pupation occurs inside the hollowed grain.
4. Pupal stage: The pupa is exarate (appendages free) and lies within the grain cavity. The pupal period lasts about 6–8 days.
5. Adult emergence: The new adult chews its way out of the grain, leaving a characteristic round exit hole. The adult lives for about 5–8 months and feeds on grains and flour.

The complete life cycle takes about 4–6 weeks (about 30–35 days) under favourable conditions, and there may be several overlapping generations per year. Development is retarded in cold weather; breeding practically stops below 15 °C. The insect survives the winter mainly as adults or larvae inside stored grains.

Nature of damage Both adults and larvae cause damage, but the larva is the more destructive stage. The larva eats out the interior of the grain, reducing it to a hollow shell; the adult also nibbles the grain and bores egg holes. Attacked grains become hollow, light and unfit for consumption or sowing. Heavy infestation produces a characteristic sour, musty smell and heating of the grain mass. The grain loses weight, nutritive value and germination capacity. The exit holes and frass (excreta) further reduce market quality. Losses in stored rice may range from 10 to 30 per cent or more in severe infestations.

Economic importance Harmful aspects:

1. It is a major primary pest of stored rice, wheat and maize, causing heavy quantitative and qualitative losses.
2. It reduces the weight, starch content and germination power of seeds, so infested grain cannot be used as seed.
3. It lowers the market value of grain and flour due to holes, dust, odour and contamination with excreta and dead insects.
4. Infested grain becomes unfit for human food and for animal feed, and may cause digestive problems if consumed.
5. It causes loss to farmers, traders, millers and the government (food reserve godowns), and the cost of control measures adds to the loss.
6. It may also transmit secondary pests and fungi into the grain mass.

Useful aspects:

1. It serves as food for insectivorous birds, lizards and predatory insects.
2. It is an important laboratory animal for research on insect physiology, genetics and pest-control methods.
3. It is used as a model organism in studies of insecticide resistance and biological control.

Control measures

1. Preventive: Store only clean, dry grain (moisture below 12–14 per cent) in airtight containers; fumigate godowns before storage; keep warehouses clean and well ventilated.
2. Physical: Sun-drying of grain, heating, and use of inert dusts or ash.
3. Mechanical: Sieving and winnowing to remove infested grains and insects.
4. Chemical: Dusting with malathion or other approved insecticides; fumigation with phosphine or methyl bromide in sealed godowns.
5. Biological: Use of natural enemies such as Anisopteromalus calandrae.
6. Resistant varieties: Storage of weevil-resistant or hard-grained varieties.

Unio (freshwater mussel, class Bivalvia) is a ciliary filter feeder. Its alimentary canal is a complete tube with no head, radula or jaws:

mouth → short oesophagus → stomach → midgut (intestine) → rectum → anus

Structure of the alimentary canal

1. Mouth. A transverse slit at the anterior end of the visceral mass, just behind the anterior adductor muscle. It lies between two pairs of flap-like labial palps, whose ciliated inner surfaces are ridged and sort food particles.
2. Oesophagus. A short, ciliated, narrow tube leading upward and backward into the stomach.
3. Stomach. A thin-walled, rounded chamber embedded in the visceral mass. It has a ciliated lining and a sorting area (ridged region). A style sac opens from it and holds the crystalline style, a transparent gelatinous rod containing digestive enzymes, which projects into the stomach. A pair of digestive glands (liver, brownish-green) opens into the stomach by ducts.
4. Midgut (intestine). A long, coiled tube that runs into the visceral mass and foot. It has a ciliated lining and a longitudinal fold, the typhlosole, that increases the surface for absorption. The style sac is continuous with its first part.
5. Rectum. The intestine leaves the visceral mass, passes through the pericardium and through the ventricle of the heart, runs over the posterior adductor muscle, and ends as the rectum.
6. Anus. Opens into the exhalant (suprabranchial) chamber just behind the posterior adductor, so faeces are carried out with the exhalant current.

Mode of digestion

• Feeding. Cilia on the gills create a water current entering by the inhalant siphon. Plankton and detritus are trapped in mucus on the gills and passed forward to the labial palps. The palps sort the particles: heavy or large ones are rejected, fine ones go to the mouth.
• Rotation of the style. Cilia in the style sac rotate the crystalline style. The rotating style winds the mucus string with food into the stomach, grinds the food, and slowly dissolves to release enzymes (amylase, cellulase, and some lipase).
• Extracellular digestion. Begins in the stomach, mainly of carbohydrates, and continues in the intestine.
• Sorting. Ridges of the stomach pass fine particles to the ducts of the digestive glands and send coarse particles to the intestine.
• Intracellular digestion. Fine particles enter the digestive glands, where the cells engulf (phagocytose) them and digest them inside food vacuoles. Wandering phagocytes also take part.
• Absorption. Digested food is absorbed in the digestive glands and the intestine and passes into the blood sinuses. The typhlosole increases the absorbing area.
• Egestion. Indigestible matter is moulded into faecal pellets in the rectum and expelled through the anus.

Larval Forms in Echinodermata

Echinoderms show radial cleavage and indeterminate development. The embryo becomes a free-swimming, bilaterally symmetrical larva, which later metamorphoses into the radially symmetrical adult. Larvae are important in phylogeny because they show that echinoderms descended from bilateral ancestors and are related to Hemichordata and Chordata. The main larval forms are:

1. Dipleurula larva (hypothetical ancestral larva)

It is not found in any living echinoderm, but is regarded as the ancestral form from which the other larvae are derived. It is bilaterally symmetrical, oval, and has a continuous ciliated band, a complete gut (mouth, stomach, anus) and three pairs of coelomic sacs. It resembles the tornaria larva of Hemichordata.

2. Bipinnaria larva (Asteroidea, starfish)

It is the first larval stage of starfish. It is free-swimming and feeds on plankton. The body has a pre-oral and a post-oral ciliated band that form arm-like lobes, and has a complete digestive tract with coelomic sacs. It later changes into the brachiolaria.

3. Brachiolaria larva (Asteroidea)

It develops from the bipinnaria by the addition of three short brachiolar arms at the anterior end, with an adhesive disc between them. The larva attaches to the substratum by these arms and then metamorphoses into a young starfish.

4. Auricularia larva (Holothuroidea, sea cucumber)

It is elongated and ear-shaped, with a single, continuous, sinuous ciliated band. It is free-swimming and feeds on plankton. It changes into the doliolaria.

5. Doliolaria larva (Holothuroidea and Crinoidea)

It is barrel-shaped with three to five transverse ciliated rings instead of a continuous band, and has a tuft of cilia at the anterior end. It is non-feeding. In holothurians it follows the auricularia, and in crinoids (sea lilies) it is the characteristic free-swimming larva, which settles on a substratum and becomes a stalked pentacrinoid.

6. Pentacula larva (Holothuroidea)

It is the last larval stage of sea cucumbers, formed from the doliolaria. Five primary tentacles appear around the mouth, the larva settles, and radial symmetry begins to develop.

7. Ophiopluteus larva (Ophiuroidea, brittle star)

It has four pairs of long ciliated arms supported by calcareous rods. It is free-swimming and feeds on plankton before metamorphosis into a young brittle star.

8. Echinopluteus larva (Echinoidea, sea urchin)

It has six pairs of ciliated arms supported by calcareous rods, and is free-swimming and planktonic. It metamorphoses into the young sea urchin.

Summary table

Significance

1. Phylogeny. Bilateral larvae, similar to the tornaria of Hemichordata, show that radial symmetry of adults is secondary and that echinoderms are deuterostomes related to chordates.
2. Dispersal. Free-swimming larvae spread sedentary or slow-moving adults over wide areas.
3. Metamorphosis. Bilateral larva changes into a radial adult, and the water vascular system develops from the left coelom.

Taxonomic Keys for Identification of Insects

A taxonomic key is a systematic device used to identify an unknown organism by presenting a series of alternative choices (couplets) based on contrasting morphological characters. For insects, keys are the basic tools of identification used in taxonomy, agriculture, and pest management.

Types of Taxonomic Keys

1. Dichotomous key (most common) At each step the user is given two mutually exclusive statements (a couplet); choosing one leads to the next couplet until the name of the taxon is reached. It may be:

• Indented (yoked) key — the two alternatives are written one below the other with indentation, so each choice is followed immediately by its consequence. Easy to follow but occupies more space.
• Bracketed (parallel) key — the two alternatives are placed side by side and numbered; the user jumps to the number indicated. Compact, but requires careful back-tracking.

2. Polyclave (multi-access) key Many characters are listed in a table or card system, and the specimen is identified by eliminating taxa that do not match the observed characters. Useful when some structures are missing or damaged.

3. Computer-aided / interactive keys Digital databases (e.g., DELTA, Lucid) allow identification from any character in any order, with images and illustrations.

Characters Used in Insect Keys

Keys rely on stable, easily observable characters such as:

• Number and type of wings (membranous, scaly, halteres)
• Mouthpart type (chewing, piercing-sucking, siphoning, sponging)
• Antennae (filiform, serrate, clavate, geniculate)
• Legs (cursorial, saltatorial, raptorial, natatorial)
• Metamorphosis (complete or incomplete)
• Segmentation of abdomen, presence of cerci, ovipositor, etc.

Merits and Limitations

Merits: Simple, quick, and usable even by non-specialists; leads step by step to a definite name; can be made for any taxonomic level (order, family, genus, species).

Limitations: Works only for the stage/sex for which it is constructed; damaged or immature specimens may not key out; a wrong choice at any couplet leads to a wrong identification; keys become outdated as classification changes.

Conclusion

Taxonomic keys, especially dichotomous keys based on morphological characters, are the principal practical tool for identifying insects. They must be used with complete, well-preserved specimens and, where possible, confirmed with descriptions, illustrations, and reference collections.

Different modes of locomotion in protozoa

Protozoa are unicellular, animal-like protists that move by specialised locomotory organelles. Four main modes are recognised:

1. Amoeboid movement (pseudopodial) Seen in Amoeba, Entamoeba. The cell pushes out a temporary cytoplasmic projection called a pseudopodium by the sol–gel transformation of cytoplasm (solation at the advancing end, gelation at the rear). The pseudopodium anchors to the substratum and the body flows into it. It is also used for engulfing food (phagocytosis).

2. Ciliary movement Seen in Paramecium, Balantidium. Numerous short, hair-like cilia cover the body surface. They beat in a coordinated metachronal rhythm (one after another, like a wave), pushing the organism forward and also sweeping food particles into the oral groove. Cilia arise from basal granules (kinetosomes).

3. Flagellar movement Seen in Euglena, Trypanosoma, Trichomonas. One or a few long, whip-like flagella beat in a spiral or undulating manner, pulling or pushing the body through water. The flagellum has a typical 9 + 2 axoneme structure and works with a neuromotor apparatus.

4. Gliding (ciliary/undulating) movement Seen in gregarines and some sporozoans. The organism glides slowly over the substratum without any visible organelle, probably by rhythmic contractions of the body wall or by secretion of a slime track. Some ciliates also show spiral or rolling movement due to the oblique beat of cilia.

Conclusion: The mode of locomotion depends on the locomotory organelle present — pseudopodia in Sarcodina, flagella in Mastigophora, cilia in Ciliophora — and is closely related to the organism's habitat and mode of nutrition.

Obelia is a colonial marine hydrozoan (phylum Cnidaria) showing metagenesis, an alternation of an asexual polyp (hydroid) colony with a sexual medusa.

1. Asexual reproduction (by the polyp colony)

• Budding. Buds arise as outgrowths of the body wall (ectoderm and endoderm), develop a mouth and tentacles, and either remain attached to form branches of the colony or separate as new zooids.
• Fragmentation and regeneration. Pieces of the colony regenerate into complete colonies because cnidarians have high regenerative power.
• Polymorphic zooids. Budding produces hydranths (feeding polyps) and gonangia (reproductive zooids), all sharing a common gastrovascular cavity.
• Medusa budding. Inside each gonangium the central blastostyle buds off medusae asexually. They are released through the opening of the gonotheca.

2. Sexual reproduction (by the medusa)

• Sexes are separate (dioecious). Each medusa has four interradial gonads (testes or ovaries) on the radial canals.
• Gametes are shed into sea water, and fertilisation is external.
• The zygote undergoes holoblastic cleavage, forming a morula, then a blastula, then a ciliated free-swimming planula larva. The planula settles, attaches, and grows into a polyp that forms a new hydroid colony.

3. Metagenesis

The polyp colony reproduces asexually and produces medusae, and the medusa reproduces sexually and produces the planula that gives rise to a new colony. The two generations alternate regularly in the life cycle.

Control measures of Echinococcus granulosus

Echinococcus granulosus is a cyclophyllidean tapeworm whose adult stage lives in the small intestine of dogs (the definitive host), while the larval stage (hydatid cyst) develops in sheep, cattle, pigs and accidentally in humans (intermediate hosts). Because the infection spreads through the dog–sheep–dog cycle and through contaminated food, water and soil, control must be directed at all three links of the cycle.

1. Control of the definitive host (dogs)

• Regular deworming of all dogs with praziquantel (5 mg/kg body weight) or arecoline hydrobromide at intervals of 6 weeks to 3 months.
• Stray dogs should be eliminated or impounded, and their number kept under control.
• Dogs should be kept tied or confined and must not be allowed to roam freely around slaughterhouses and grazing land.
• Feeding of dogs with raw offal, especially sheep liver and lungs containing hydatid cysts, must be strictly prohibited. Offal should be boiled or incinerated before being given to dogs.

2. Control of the intermediate host (sheep, cattle, goats, pigs)

• Infected animals should not be slaughtered for food; their infected organs must be condemned and either buried deep or burnt, never thrown to dogs.
• Slaughtering should be carried out only in authorised abattoirs under proper meat inspection so that infected organs are detected and destroyed.
• Proper grazing management should be followed so that livestock do not graze on land heavily contaminated with dog faeces.

3. Control of the source of infection and environment

• Dog faeces should be collected and disposed of safely (buried or burnt); it should never be used as manure on pasture or vegetable gardens.
• Safe drinking water supply and proper sanitary latrines should be provided to prevent faecal contamination of food and water.
• Health education of the public about the mode of transmission, the danger of close contact with dogs, and the importance of washing hands and vegetables before eating.

4. Control in humans

• Personal hygiene: washing hands with soap after handling dogs and before eating.
• Vegetables and fruits should be washed thoroughly or cooked.
• Surgical removal of hydatid cysts, and treatment with albendazole or mebendazole in inoperable cases.
• Serological screening (e.g. ELISA) of high-risk groups such as shepherds and slaughterhouse workers for early detection.

5. Administrative and surveillance measures

• Compulsory notification and recording of hydatid disease cases.
• Periodic meat inspection and faecal examination of dogs in endemic areas.
• Coordinated "One Health” approach involving veterinary and public health departments to break the dog–sheep–dog cycle.

In short, the control of E. granulosus depends mainly on breaking the dog–sheep–dog cycle by deworming dogs, preventing dogs from eating infected offal, and safe disposal of infected organs and dog faeces, supported by public health education.

The thorax of the freshwater prawn (Palaemon, Macrobrachium) is fused with the head as the cephalothorax, covered by the carapace, and bears eight pairs of thoracic appendages (thoracopods): three pairs of maxillipeds (thoracopods 1 to 3) and five pairs of pereiopods or walking legs (thoracopods 4 to 8). Basically each is biramous, with a protopodite (coxa and basis), an endopodite and an exopodite, but exopodites are lost in the pereiopods.

A. Maxillipeds (food handling)

1. First maxilliped. Flattened and leaf-like. The coxa bears an epipodite and the basis has endites. It has a small exopodite and a slender, unsegmented endopodite. It holds food near the mouth.
2. Second maxilliped. Shorter, with a 5-segmented leg-like endopodite and a long exopodite with a flagellum. It has an epipodite and a gill. It handles food and helps to create the respiratory current.
3. Third maxilliped. Pediform (leg-like), with a 5-segmented endopodite and a small exopodite. It handles food and also cleans the gills.

B. Pereiopods (walking legs)

Each has seven segments: coxa, basis, ischium, merus, carpus, propodus and dactylus. They are uniramous, and the exopodite is absent. 4. First pereiopod. Slender and chelate. It picks up food and cleans the antennae. 5. Second pereiopod. Longest and strongest, and chelate. It is used for offence and defence, and for seizing prey. It is larger in males. 6. Third, fourth and fifth pereiopods. Slender, non-chelate walking legs ending in a pointed dactylus. They are used for walking and crawling over the bottom, and the third also assists in cleaning.

Summary

All the appendages are serially homologous and are modified for different functions (division of labour).

Social behavior in insects (also called sociality or eusociality) refers to the cooperative living of individuals of the same species in an organized colony, where members live together, share a common nest, cooperate in caring for the young, and divide labour among specialized groups called castes. It is best developed in the orders Hymenoptera (ants, bees, wasps) and Isoptera (termites).

Characteristics of true social (eusocial) insects:

1. Cooperative brood care — young ones are reared collectively by members other than the parents.
2. Reproductive division of labour — only one or a few individuals (queen/king) reproduce, while others (workers, soldiers) remain sterile.
3. Overlapping generations — offspring of one generation remain in the colony and help rear the next generation.
4. Common nest — all members live together in a common, well-defined nest.
5. Caste system — morphologically and behaviorally distinct castes (queen, workers, soldiers, drones) perform different duties.

Examples of social insects:

• Honey bee (Apis): colony has one queen (egg-laying), many sterile female workers (nursing, foraging, cleaning, guarding) and drones (mating).
• Ants (Formica, Atta): queen, sterile workers and soldiers; some species cultivate fungi or keep aphids.
• Termites (Termes): reproductive king and queen plus sterile worker and soldier castes; soldiers defend the colony with large mandibles.
• Wasps (Polistes, Vespa): primitively social, with a queen and workers.

Advantages of social life:

• Division of labour increases efficiency in food collection, nest building and defence.
• Better protection from enemies and harsh environmental conditions.
• Cooperative brood care raises survival of the young.
• Large colonies can store food and regulate temperature (e.g., honey bees cluster to keep the hive warm).

Disadvantages:

• Competition for food and space within the colony.
• Easy spread of diseases and parasites in crowded nests.
• Sterile workers sacrifice their own reproduction, so the colony depends on the queen; loss of the queen can destroy the colony.

Conclusion: Social behavior in insects is a highly evolved form of cooperation in which genetic relatedness (kin selection) and division of labour maintain a well-organized colony, giving such insects a great advantage in survival and reproduction.

Diversity of Molluscs in Nepal (5 marks)

1. General status. Nepal, though a small landlocked country, has a rich molluscan fauna due to its wide altitudinal range (about 60 m to above 5000 m), varied climate, and dense network of rivers, streams, lakes, ponds, and moist forests. About 200–250 species of molluscs have been reported from Nepal, and new records are still being added. They belong mainly to two classes: Gastropoda (snails and slugs) and Bivalvia/Pelecypoda (clams and mussels). A few records of Scaphopoda also exist.

2. Class Gastropoda (largest group). This is the most diverse class in Nepal, with both aquatic and terrestrial forms:

• Terrestrial snails and slugs: Helix, Achatina (giant African snail), Pila (apple snail), and many endemic high-altitude snails of the families Pupillidae, Enidae, and Ariophantidae. The Himalayan region is a hotspot for land snails; several species are endemic to Nepal.
• Freshwater snails: Vivipara (river snail), Melania, Bithynia, Lymnaea, Gyraulus, Indoplanorbis, and Biomphalaria. These occur in ponds, lakes (Phewa, Rara, Begnas), and slow-flowing rivers.
• Slugs: Laevicaulis, Mariaella, and Anadenus are found in moist forests of the hills and Terai.

3. Class Bivalvia (Pelecypoda). Freshwater bivalves are represented by unionid mussels (Lamellidens, Parreysia) and freshwater clams (Corbicula, Pisidium, Sphaerium). They occur in rivers such as the Koshi, Gandaki, Karnali, and their tributaries, and in lakes and ponds. Marine bivalves are absent because Nepal has no coastline.

4. Endemism and ecological importance. Nepal has a notable degree of endemism among land snails of the mid-hills and Himalayas. Molluscs serve as indicators of water quality, act as intermediate hosts of parasites (e.g., Lymnaea for Fasciola, Biomphalaria for Schistosoma), help in decomposition and nutrient cycling, and are used as food and in traditional medicine in some communities.

5. Threats and conservation. Habitat destruction, water pollution, deforestation, pesticide use, and climate change threaten molluscan diversity. Systematic surveys and conservation measures are needed, as many species remain poorly documented.

Conclusion. Nepal's molluscan fauna is moderately diverse, dominated by gastropods, with notable endemism in the Himalayas and significant ecological and economic importance.

Pearls are smooth, rounded, lustrous concretions produced inside the soft body of certain molluscs (mainly pearl oysters and mussels). They are composed chiefly of calcium carbonate (CaCO₃) in the form of aragonite, cemented together by an organic protein called conchiolin, and are secreted by the mantle of the mollusc. Pearls are valued as gems and are also called "organic gems."

Formation of pearls:

1. Irritant enters the mantle: A foreign particle such as a grain of sand, a parasite, or a piece of broken shell lodges between the mantle and the shell of the oyster. This acts as an irritant.

2. Mantle secretes nacre: To protect itself from the irritant, the mantle epithelium secretes nacre (mother-of-pearl) in concentric layers around the particle. Nacre consists of thin alternating layers of aragonite (CaCO₃) and conchiolin.

3. Layer-by-layer deposition: The nacre is deposited continuously in thin successive layers, each layer being added over the previous one. This gives the pearl its characteristic pearly lustre (iridescence) due to interference of light from the overlapping layers.

4. Formation of the pearl sac: The mantle tissue forms a sac-like structure called the pearl sac around the irritant. The cells of this sac continue to secrete nacre inward, gradually building up the pearl.

5. Growth and completion: Over months to years (usually 2–5 years), the irritant becomes completely coated with many layers of nacre, forming a solid, rounded pearl. The size of the pearl depends on the size of the irritant and the duration of nacre deposition.

Types:

• Natural pearls – formed accidentally in wild oysters.
• Cultured pearls – formed when humans deliberately insert an irritant (nucleus) into the oyster, e.g., in Japan and Australia.

Chemical composition: About 82–86% CaCO₃ (aragonite), 10–14% conchiolin (organic matter), and 2–4% water.

Echinoderms are an ancient, exclusively marine phylum. Their origin is explained as follows.

1. Time of origin. They first appear in the Early Cambrian (about 540 million years ago); the fossil record is rich because of their calcareous endoskeleton.
2. Bilateral ancestor. The larvae (bipinnaria, auricularia, etc.) and the earliest fossil forms are bilaterally symmetrical. Echinoderms therefore arose from a free-swimming, bilaterally symmetrical ancestor, and the radial symmetry of the adult is secondary, an adaptation to a sedentary or slow-moving life.
3. Deuterostome affinities. Radial cleavage, enterocoelic coelom formation, the mouth forming away from the blastopore, and the mesodermal skeleton link echinoderms with Hemichordata and Chordata in the Deuterostomia. The dipleurula larva resembles the tornaria of Hemichordata.
4. Fossil radiation. Early fossil groups include Carpoidea (Homalozoa), Cystoidea, Blastoidea and Edrioasteroidea, mostly extinct. Crinoidea, Asteroidea, Ophiuroidea and Echinoidea appear by the Ordovician. Holothuroidea are known mostly from spicules. Roughly 6,000 species are living and a larger number are extinct.
5. Trends. Evolution moved from a free-moving bilateral form to a fixed or slow radial form, with the evolution of the water vascular system, tube feet and a calcareous endoskeleton.

Conclusion. Echinoderms originated in the Early Cambrian from a bilaterally symmetrical deuterostome ancestor related to the chordate line, and radial symmetry developed later.

Affinities of Balanoglossus

Balanoglossus (acorn worm) is a hemichordate whose affinities are traced in three directions:

1. With Annelida (resemblance): Body is soft, vermiform and burrowing; the body is divided into proboscis, collar and trunk, and the trunk shows annulations. It has a coelom, a contractile muscular body wall and a similar mode of burrowing life. But it differs in having gill slits, a stomochord and a dorsal nerve cord, so the resemblance is only superficial.
2. With Chordata (true affinity): It possesses the three basic chordate features — (i) a dorsal, hollow, tubular nerve cord (in the collar region), (ii) pharyngeal gill slits, and (iii) a notochord-like stomochord in the proboscis. Hence it is placed in the phylum Hemichordata, regarded as a subphylum of Chordata or as a link between invertebrates and chordates.
3. With Echinodermata (closest affinity): The tornaria larva of Balanoglossus closely resembles the bipinnaria larva of echinoderms; both have a similar ciliated band and coelomic sacs. The stomochord and the water-vascular system are also comparable. Therefore Hemichordata and Echinodermata are grouped as sister phyla (Ambulacraria).

Thus Balanoglossus is a connecting link between Annelida, Echinodermata and Chordata.

Affinities of Herdmania

Herdmania is a marine ascidian (urochordate) showing clear chordate affinities:

1. Chordate characters: The free-swimming tadpole larva possesses all three chordate features — a notochord in the tail, a dorsal hollow nerve cord and pharyngeal gill slits. The adult retains gill slits in the pharynx and has a well-developed endostyle, so it is a true chordate.
2. With other chordates: The presence of a pharynx with gill slits, an endostyle, a dorsal tubular nerve cord and a post-anal tail in the larva links it with Cephalochordata and Vertebrata. The endostyle corresponds to the thyroid gland of vertebrates.
3. With invertebrates (retrogressive metamorphosis): The adult loses the notochord, nerve cord and tail, becoming sessile and sac-like, which gives it a superficial resemblance to Porifera and Coelenterata. But this is due to retrogressive metamorphosis, not true affinity.

Thus Herdmania is undoubtedly a chordate, placed in Urochordata, and its adult simplicity is secondary and adaptive.

a. Principles of ICZN

The International Code of Zoological Nomenclature rests on four main principles: Binomial nomenclature (every species gets a two-part Latin name, genus + species), Priority (the oldest available name is the valid one), Homonymy (the same name cannot be used for two different taxa) and Synonymy (a taxon may have several names but only one is valid). Names must be Latinised or treated as Latin, and type specimens fix the identity of each named taxon.

b. Osmoregulation in protozoa

Freshwater protozoa live in a hypotonic medium, so water enters by osmosis. They expel excess water and dissolved wastes through contractile vacuoles, which fill, fuse with the cell membrane and discharge to the outside. The vacuole works by active transport of ions (mainly Na⁺) into it, making it hypertonic so water follows osmotically. Marine protozoa are isotonic with their medium and usually lack contractile vacuoles.

c. Polyps and medusa

These are the two body forms of Cnidaria. A polyp is cylindrical, sessile, with the mouth and tentacles directed upward and the aboral end attached to a substratum (e.g. Hydra, Obelia). A medusa is umbrella- or bell-shaped, free-swimming, with the mouth and tentacles directed downward on the concave subumbrellar side (e.g. Aurelia). Polyps reproduce asexually by budding; medusae reproduce sexually. Many cnidarians show alternation of these two forms (metagenesis).

d. Morphological adaptations of parasitic helminths

Parasitic helminths show: loss of locomotory and digestive organs (they absorb pre-digested food through the body surface); degeneration of sense organs; development of suckers, hooks, rostellum, and spines for attachment (e.g. Taenia, Fasciola); thick protective cuticle resistant to host enzymes; enlarged reproductive organs with enormous egg output; hermaphroditism (in most trematodes and cestodes) ensuring fertilization even when solitary; and anaerobic respiration suited to the oxygen-poor gut.

e. Adaptive radiations in Polychaeta

Polychaetes show adaptive radiation into many habitats and modes of life: errant forms (e.g. Nereis) with well-developed parapodia for crawling and swimming; sedentary tube-dwellers (e.g. Sabella, Chaetopterus) with reduced parapodia and feathery branchiae; burrowers (e.g. Arenicola) with reduced parapodia and gills; pelagic forms (e.g. Tomopteris) with flattened, paddle-like parapodia; and commensal/parasitic forms with reduced segmentation. This radiation reflects exploitation of different niches in marine environments.

f. Pheromones in insects

Pheromones are chemical messengers secreted by insects to communicate with members of the same species. Examples: sex pheromones (e.g. bombykol of female silkworm moth attracts males), aggregation pheromones (bark beetles), alarm pheromones (aphids), trail pheromones (ants), and queen substance in honeybees that suppresses ovary development in workers. They are highly specific and effective at extremely low concentrations.

g. Mouthparts of cockroach

The mouthparts of Periplaneta are of the biting and chewing type and consist of: labrum (upper lip), a pair of mandibles (strong toothed jaws), a pair of maxillae (each with galea, lacinia, and palp), labium (lower lip with palps), and a median hypopharynx (tongue). They are adapted for chewing food and are surrounded by the preoral cavity.

h. Shells in Gastropoda

The gastropod shell is a single, spirally coiled, calcareous structure secreted by the mantle. It may be dextral (right-coiled) or sinistral (left-coiled). It consists of an apex, whorls, suture, body whorl, aperture, and operculum in some forms. In slugs and some limpets the shell is reduced or absent. The shell protects the soft body and shows torsion during development.

i. Gastrotricha

Gastrotricha are microscopic, aquatic, worm-like pseudocoelomates with a ventral ciliated epidermis (hence "hairy-bellied"). The body is covered by a cuticle bearing scales and spines. They have a complete digestive tract, a protonephridial system, and are hermaphroditic. They live in freshwater and marine sediments, gliding over surfaces by ciliary action.

j. Origin of protochordates

Protochordates (Hemichordata, Urochordata, Cephalochordata) are regarded as the forerunners of vertebrates. Their origin is traced from dipleurula-like ancestral echinoderm larvae by the enterocoelous formation of coelom and the development of gill slits, a notochord, and a dorsal hollow nerve cord. The auricularia larva of echinoderms resembles the tornaria larva of hemichordates, supporting an echinoderm–chordate relationship. Thus protochordates arose from a sessile or semi-sessile ancestor that later gave rise to free-swimming chordates.