Babesia bigemina — Life Cycle, Pathogenicity and Control Measures

1. Systematic Position

Babesia bigemina is an intraerythrocytic protozoan parasite of cattle that causes bovine babesiosis (redwater fever, Texas fever, tick fever). It is transmitted by ticks of the genus Boophilus (Rhipicephalus).

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2. Life Cycle

B. bigemina has a digenetic life cycle requiring two hosts:

• Definitive host / vector — the tick (Boophilus microplus, B. annulatus, B. decoloratus)
• Intermediate host — cattle (and other bovines)

The life cycle is completed in three phases:

(a) Asexual phase in the vertebrate host (cattle)

1. An infected tick injects sporozoites into the blood of cattle while feeding.
2. Sporozoites enter erythrocytes and become rounded trophozoites.
3. Trophozoites grow and divide by binary fission (merogony) into merozoites, producing pairs of pear-shaped bodies joined at an acute angle — the characteristic paired pyriform stage.
4. Repeated merogony destroys RBCs, releasing merozoites that invade fresh erythrocytes.
5. Some merozoites develop into gametocytes (round or oval forms), which are the stage infective to the tick.

(b) Sexual phase in the tick (vector)

6. When a tick feeds on an infected animal, it takes up gametocytes along with blood.
7. In the tick's gut, gametocytes develop into gametes, which fuse to form a zygote.
8. The zygote becomes a motile kinete (vermicule), which penetrates the gut wall and migrates to the ovary and other tissues.

(c) Sporogony and transovarial transmission

9. Kinetes enter the ova (eggs) of the female tick — this is transovarial transmission.
10. Inside the developing egg/larva, kinetes undergo sporogony to produce sporozoites.
11. The infected larval tick (and later nymph/adult) carries sporozoites in its salivary glands and transmits them to the next cattle host during feeding.

Key points: Transovarial transmission makes the tick itself an infective reservoir; larvae hatching from infected eggs can transmit the parasite without ever having fed on an infected animal. There is no transstadial loss — infection passes from larva → nymph → adult.

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3. Pathogenicity

Mode of infection

Infected larval, nymphal or adult ticks transmit sporozoites during blood feeding. In Nepal, outbreaks occur mainly in the summer and rainy season, when tick activity is highest.

Incubation period

About 7–21 days after tick bite.

Pathogenesis

1. Invasion and destruction of erythrocytes — merozoites multiply inside RBCs, causing massive haemolysis.
2. Anaemia — both from direct RBC destruction and from immune-mediated haemolysis.
3. Haemoglobinaemia and haemoglobinuria — free haemoglobin in plasma and urine gives the characteristic dark red to coffee-coloured urine (redwater).
4. Jaundice — due to excessive bilirubin from haemolysis.
5. Fever — release of pyrogens from destroyed cells; temperature rises to 40–41.5 °C.
6. Hypoxia and organ damage — anaemia plus sludging of parasitised RBCs in capillaries causes damage to brain, kidney and liver.
7. Cerebral involvement — in severe cases, nervous signs (incoordination, convulsions) appear, often fatal.

Clinical signs in cattle

• High fever (40–41.5 °C), dullness, inappetence
• Rapid, weak pulse and laboured breathing
• Pale to icteric mucous membranes
• Dark red or coffee-coloured urine
• Drop in milk yield; abortion in pregnant cows
• Constipation followed by diarrhoea
• Death within 1–2 weeks in untreated cases; mortality may reach 50–90 % in susceptible herds

Post-mortem findings

• Enlarged, pale, jaundiced liver; distended gall bladder with thick bile
• Enlarged spleen with dark pulp
• Kidneys swollen and dark (haemoglobin-stained)
• Blood thin and watery; bladder filled with red urine

Susceptibility

• Calves below 6–9 months are relatively resistant (inverse age resistance).
• Exotic and crossbred cattle are more susceptible than indigenous Bos indicus breeds.
• Animals recovered from infection develop premunity (sterile immunity maintained by repeated exposure).

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4. Control Measures

(a) Tick control (most important)

1. Acaricide dipping/spraying — use of deltamethrin, cypermethrin, amitraz or organophosphates at regular intervals (every 2–3 weeks during tick season).
2. Rotation of acaricides to prevent resistance.
3. Pasture management — avoid heavily tick-infested pastures; rest pastures for several weeks.
4. Manual removal of ticks on small herds; keep tick-free sheds.

(b) Chemoprophylaxis and treatment

5. Treatment of affected animals:
   • Diminazene aceturate (Berenil) @ 3.5 mg/kg body weight IM — drug of choice
   • Imidocarb dipropionate @ 1.2 mg/kg SC
   • Oxytetracycline @ 10 mg/kg for supportive action
6. Supportive therapy — blood transfusion in severe anaemia, fluid therapy, iron and vitamin B-complex supplements, antipyretics.
7. Chemoprophylaxis in endemic areas — imidocarb or diminazene at lower doses during outbreak season.

(c) Immunization

8. Vaccination with attenuated B. bigemina (live attenuated vaccine) or killed antigen vaccine in endemic areas.
9. Premunization — deliberate infection of susceptible animals followed by treatment to induce immunity (used in some countries).

(d) Management practices

10. Quarantine of newly purchased animals before mixing with the herd.
11. Segregation of infected animals; provide tick-free grazing.
12. Breeding for resistance — prefer indigenous Bos indicus breeds which show natural resistance.
13. Regular health monitoring and prompt reporting to veterinary authorities.

(e) Public health / environmental measures

14. Awareness programmes for farmers about tick-borne diseases.
15. Proper disposal of infected carcasses and tick-infested materials.
16. Vector surveillance to detect tick population density and species composition.

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5. Summary

Babesia bigemina is a tick-borne intraerythrocytic protozoan causing bovine babesiosis. Its life cycle involves asexual multiplication in cattle and sexual reproduction with transovarial transmission in Boophilus ticks. The parasite causes fever, anaemia, jaundice and haemoglobinuria, with high mortality in exotic cattle. Control depends on tick eradication by acaricides, chemotherapy with diminazene/imidocarb, vaccination and good herd management.

Schistosoma haematobium is a digenetic trematode (blood fluke) that causes urinary schistosomiasis (bilharziasis) in humans. It is dioecious (separate sexes), unlike most other trematodes which are hermaphroditic.

Structure

Adult worms

• Sexes separate; the male is broader (about 10–15 mm long) and the female is slender and longer (about 15–20 mm).
• The male body is folded lengthwise to form a gynaecophoric canal, in which the female lies during copulation and for most of adult life.
• Suckers: an anterior oral sucker surrounding the mouth and a ventral sucker (acetabulum) behind it, used for attachment to the venous wall.
• Body surface (tegument): covered with tuberculate spines (especially in the male), which help in attachment and movement against the blood flow.
• Digestive system: mouth → oesophagus → bifurcated intestinal caeca; no anus (blind-ending).
• Excretory system: flame cells → collecting tubules → excretory pore.
• Reproductive system:
  • Male: testes (4–5 in S. haematobium), vas deferens, seminal vesicle, cirrus.
  • Female: single ovary, oviduct, vitelline glands, ootype, and a short uterus opening at the genital pore.
• Nervous system: paired cerebral ganglia with nerve cords.
• Habitat: vesical and pelvic venous plexuses around the urinary bladder (hence the name haematobium — blood-loving).

Larval stages

• Miracidium: ciliated, free-swimming, infects the snail.
• Sporocyst (mother and daughter): sac-like, inside the snail, produces cercariae asexually.
• Cercaria: free-swimming, forked-tailed larva that infects humans by penetrating the skin.

Life cycle

Definitive host: man (adults live in vesical veins). Intermediate host: freshwater snail of the genus Bulinus.

1. Eggs (terminal-spined, about 110–170 µm) are laid in the venules of the bladder wall and pass out in urine.
2. In fresh water, eggs hatch into miracidia within minutes to hours.
3. The miracidium penetrates the snail (Bulinus) and develops into a mother sporocyst, then daughter sporocysts, and finally cercariae (asexual multiplication).
4. Cercariae escape from the snail into water and swim freely.
5. They penetrate human skin (usually of the feet or legs) when the person wades or bathes in infected water; the tail is shed and the larva becomes a schistosomulum.
6. Schistosomula enter the lymphatics and blood, pass through the heart → lungs → heart → systemic circulation, and finally reach the vesical venous plexus of the bladder.
7. In the venules they mature into adult males and females, pair up (female in the gynaecophoric canal), and begin egg production.
8. Eggs are laid in the venules, penetrate the bladder wall, and are voided in urine, completing the cycle.

Pathogenesis: Eggs lodged in the bladder wall cause granulomatous inflammation, haematuria, dysuria, and fibrosis; chronic infection may lead to squamous cell carcinoma of the bladder.

Control: treatment with praziquantel, avoidance of infected water, snail control, and proper sanitation.

Reproductive Organs of Leech (Hirudinaria granulosa)

The leech is a hermaphrodite (bisexual) animal — both male and female reproductive organs are present in the same individual. The reproductive system is well developed and occupies a large part of the body cavity. It consists of male reproductive organs, female reproductive organs and accessory reproductive organs.

A. Male Reproductive Organs

1. Testes (Testis sacs): There are 11 pairs of small, rounded, whitish testes. They are located in the middle of the body, one pair in each segment from the 12th to the 22nd segment. Each testis lies inside a thin-walled sac called the testis sac.

2. Vasa efferentia: From each testis, a short duct called the vas efferens arises. The vasa efferentia of each side unite to form a longitudinal duct called the vas deferens.

3. Vas deferens: There are two vasa deferentia (one on each side) running longitudinally. They are highly coiled and serve to carry spermatozoa.

4. Epididymis: Near the anterior end, each vas deferens becomes thickened and coiled to form the epididymis, which stores sperm.

5. Ejaculatory duct: The epididymis continues as a short, straight ejaculatory duct.

6. Atrium: Each ejaculatory duct opens into a muscular chamber called the atrium (genital atrium).

7. Male gonopore: The two atria open to the exterior through a common male gonopore situated ventrally in the 10th segment. The male gonopore is the external opening of the male reproductive system.

B. Female Reproductive Organs

1. Ovaries: There is one pair of ovaries, located in the 11th segment, one on each side of the nerve cord. Each ovary is a small, rounded, whitish body.

2. Oviducts: From each ovary arises a short duct called the oviduct. The two oviducts run longitudinally and unite to form a short common oviduct.

3. Female gonopore: The common oviduct opens to the exterior through the female gonopore, situated ventrally in the 13th segment.

C. Accessory Reproductive Organs

1. Albumen glands: A pair of lobed, whitish albumen glands opens into the common oviduct. They secrete albumen which nourishes the eggs.

2. Prostate glands: A pair of prostate glands is present. Each prostate gland opens into the ejaculatory duct (near the atrium) and secretes a fluid that forms the wall of the spermatophore, which encloses the spermatozoa during transfer.

3. Spermathecae: These are small sacs present in segments 11–13 (typically three pairs). They receive and store spermatozoa during copulation, and the stored sperm fertilizes the eggs later.

4. Clitellum (Cocoon gland): Segments 9, 10 and 11 are modified to form the clitellum, which is glandular and secretes a cocoon (capsule) for receiving the eggs during breeding season.

D. Copulation and Fertilization

Leeches are cross-fertilizing hermaphrodites. During copulation, two leeches attach to each other in opposite directions — the male gonopore of one comes in contact with the female gonopore of the other. Spermatozoa enclosed in spermatophores are transferred and stored in the spermathecae (present in segments 11–13). Fertilization is internal. After fertilization, the clitellum secretes a cocoon that slips off the body and receives the fertilized eggs. Development is direct (no larval stage).

Summary Table

Thus, the leech possesses a complete and well-organized hermaphroditic reproductive system, with the male organs being more numerous and anteriorly placed, and the female organs lying just behind them.

Piercing and Sucking Type of Mouth Parts

Introduction

The piercing and sucking type of mouth parts are found in insects that feed on the blood of vertebrates or the sap of plants. These mouth parts are modified into a sharp, needle-like structure called the proboscis, which can pierce the skin or plant tissue and suck out the fluid. This type is well developed in insects such as mosquitoes, bugs, and aphids.

Parts of Piercing and Sucking Mouth Parts

The piercing and sucking mouth parts consist of the following parts:

1. Labrum (Upper Lip)

• It is a long, narrow, and pointed structure.
• It forms the upper covering of the mouth parts.
• In some insects (e.g., mosquito), it is elongated and helps in guiding the other mouth parts during piercing.

2. Labium (Lower Lip)

• It is the largest and most prominent part.
• It forms a sheath or groove that encloses and protects the other piercing structures.
• It does not pierce the host tissue itself but acts as a supporting guide.
• In mosquitoes, the labium bends backward when the stylets enter the skin.

3. Mandibles (Upper Jaws)

• They are modified into long, slender, needle-like stylets.
• They are sharp and help in piercing the skin or plant tissue.
• In mosquitoes, they are fine and blade-like.

4. Maxillae (Lower Jaws)

• They are also modified into long, sharp stylets.
• They bear serrated edges that help in cutting and piercing the tissue.
• In mosquitoes, the maxillae are the main piercing organs and form the food canal along with the labrum.

5. Hypopharynx

• It is a median, needle-like structure lying between the mandibles and maxillae.
• It contains the salivary duct through which saliva is injected into the host.
• Saliva prevents blood coagulation and helps in the free flow of blood or sap.

6. Food Canal (Proboscis)

• The labrum and maxillae together enclose a food canal.
• Through this canal, the liquid food (blood or plant sap) is sucked up into the mouth.
• The sucking is brought about by the pharynx and cibarium, which act as a suction pump.

Diagram (Description)

The mouth parts together form a long, needle-like proboscis. The labium forms the outer sheath; inside it lie the mandibles, maxillae, hypopharynx, and labrum. When the insect bites, the labium bends back and the stylets penetrate the skin, saliva is injected through the hypopharynx, and blood is sucked through the food canal.

Examples

• Mosquito (Anopheles, Culex) — pierces vertebrate skin and sucks blood.
• Bed bug (Cimex) — blood-sucking type.
• Aphid and plant bug — pierce plant tissue and suck sap.

Conclusion

The piercing and sucking mouth parts are highly specialized structures consisting of the labrum, labium, mandibles, maxillae, and hypopharynx. Together they form a proboscis adapted for piercing the host tissue and sucking liquid food.

Water Vascular System in Asterias

The water vascular system is a unique echinoderm feature, and in Asterias (sea star) it is the main system of locomotion, food capture and respiration. It is a coelomic, seawater-filled hydraulic system derived from the larval left hydrocoel. It consists of the following parts:

1. Madreporite (sieve plate) A small, circular, calcareous, sieve-like plate on the aboral (dorsal) surface, near the junction of two arms. Its grooves bear many pores that open into the underlying water vascular system. It allows entry of seawater into the system and regulates internal water pressure.

2. Stone canal A short, S-shaped, calcareous tube running vertically downwards from the madreporite. Its wall is strengthened by calcareous rings (hence the name). It connects the madreporite with the ring canal.

3. Ring canal A circular, non-calcareous water channel lying in the perioral coelom around the oesophagus, just above the buccal membrane. It is the central distributing channel of the system. It bears:

• Tiedemann's bodies — small, yellowish, glandular sacs (usually 5 pairs) on its inner side, which produce coelomocytes that remove bacteria and foreign particles from the water.
• Polian vesicles — one or two thin-walled, sac-like reservoirs on its outer side, which store water and maintain pressure in the system.

4. Radial canals Five radial canals arise from the ring canal, one passing into each arm. Each runs along the mid-ventral line of the arm, lodged in the ambulacral groove between the two rows of ossicles. Each radial canal gives off a series of short lateral branches, the lateral canals.

5. Lateral canals and tube feet Each lateral canal opens into a tube foot. A tube foot consists of:

• Ampulla — a small, rounded, muscular sac lying above the radial canal (in the coelom of the arm).
• Podium (tube foot proper) — a hollow, muscular, cylindrical tube passing through the ambulacral groove and ending in a sucker. It bears a calcareous end-plate and is provided with valves at its base.

Working (mechanism of locomotion) Seawater enters through the madreporite → stone canal → ring canal → radial canals → lateral canals → ampullae. Contraction of the ampullary muscles forces water into the podium, which elongates and its sucker attaches to the substratum. Longitudinal muscles of the podium then contract, shortening it and pulling the body forward. Relaxation of the ampulla allows water to return, and the tube foot shortens and detaches. The tube feet of the two sides of each arm act in a coordinated manner, so the animal creeps slowly in any direction. The system also helps in capturing prey (by gripping with suckers), in respiration (through the thin wall of the podia), and in excretion of nitrogenous waste.

Functions

1. Locomotion by means of tube feet.
2. Capture and handling of food.
3. Respiration through the thin, permeable walls of the podia.
4. Excretion of nitrogenous wastes.
5. Maintenance of water pressure and body form (hydrostatic skeleton).

In short, the water vascular system of Asterias is a seawater-filled hydraulic system in which the madreporite, stone canal, ring canal, radial canals, lateral canals and tube feet work together to bring about slow creeping locomotion and also serve in feeding, respiration and excretion.

Affinities of Branchiostoma

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

1. Affinities with Invertebrates (Annelida and Mollusca)

Branchiostoma shows several features that resemble invertebrates:

• Shape and habits: Elongated, worm-like body and burrowing habit resemble annelids.
• Segmentation: Body shows metameric segmentation with myotomes, as in annelids.
• Coelom: A well-developed coelom is present, but it is enterocoelous (formed from gut pouches), unlike the schizocoelous coelom of annelids and molluscs.
• Nephridia: Excretory organs are segmentally arranged nephridia, similar to those of annelids.
• Ciliated epidermis: Single-layered ciliated epidermis resembles that of annelids.
• Ciliation: The ciliated larva and ciliary feeding currents recall the ciliated larvae of some invertebrates, though the larva is not a trochophore.
• Absence of jaws and paired fins: It lacks a definite head, jaws and paired appendages, as in invertebrates.

However, these similarities are superficial and due to convergent evolution, not true relationship.

2. Affinities with Vertebrates (Chordate Characters)

Branchiostoma is fundamentally a chordate and shows all the three basic chordate features throughout life:

• Notochord: A persistent, unsegmented notochord extends the whole length of the body and persists in the adult.
• Dorsal hollow nerve cord: Present above the notochord, with a small anterior cerebral vesicle.
• Pharyngeal gill slits: Numerous paired gill slits in the pharynx for respiration and filter feeding.

Other vertebrate-like features:

• Coelom and mesoderm: Well-developed coelom and triploblastic organisation.
• Muscle blocks (myotomes): Segmental muscle blocks as in fish.
• Blood vascular system: Closed, with a dorsal vessel, ventral vessel and branchial hearts; no true heart.
• Nervous system: Brain is not well developed; nerve cord gives off paired nerves.
• Endostyle: Homologous to the thyroid gland of vertebrates.
• Hepatic caecum: Homologous to the liver of vertebrates.
• Hatschek's pit: Homologous to the anterior lobe of the vertebrate pituitary gland.

3. Differences from Vertebrates

• No cranium, no brain case, hence Acrania.
• No jaws, no paired fins, no true head.
• Notochord persists and extends into the rostrum (hence Cephalochordata).
• Heart is absent; blood is colourless.
• Excretory organs are nephridia, not kidneys.
• Skin is naked and ciliated; no scales, no exoskeleton.

4. Systematic Position

Branchiostoma is placed in:

It is regarded as a primitive chordate that bridges the gap between invertebrates and vertebrates. It retains chordate characters in the adult but lacks the vertebrate specialisations (cranium, jaws, paired fins, true heart and kidneys).

Conclusion

Branchiostoma shows superficial resemblances to annelids and molluscs, but its true affinities lie with chordates. It is a deuterostome chordate and is considered the connecting link between invertebrates and vertebrates, being the most primitive living chordate.

Biological Species Concept in Taxonomy

Definition. The biological species concept (BSC) defines a species as a group of actually or potentially interbreeding natural populations that are reproductively isolated from other such groups. It was most clearly formulated by Ernst Mayr (1942), who stated: "Species are groups of actually or potentially interbreeding natural populations which are reproductively isolated from other such groups."

Key criteria.

1. Interbreeding — Members of the same species can mate freely under natural conditions and produce fertile offspring.
2. Gene flow — There is exchange of genes among populations of the same species, so they share a common gene pool.
3. Reproductive isolation — Members of different species do not interbreed, or if they do, the hybrids are sterile or inviable. This isolation may be prezygotic (habitat, temporal, behavioural, mechanical, gametic isolation) or postzygotic (hybrid inviability, hybrid sterility, hybrid breakdown).
4. Natural populations — The concept applies to populations in nature, not to organisms in captivity or to asexual forms.

Merits.

• It is based on gene flow and reproductive compatibility, which are objective, testable criteria.
• It explains why members of a species resemble one another and why species remain distinct.
• It gives a clear genetic and evolutionary meaning to the species category, linking speciation to the origin of reproductive barriers.

Demerits / limitations.

• Not applicable to asexual organisms — bacteria, many protists, and parthenogenetic forms do not interbreed, yet form distinct groups.
• Not applicable to fossils — reproductive behaviour of extinct forms cannot be tested.
• Hybridisation occurs in nature (e.g., mules from horse × donkey, many plants), which the concept cannot easily accommodate.
• Allopatric populations that could interbreed but never meet cannot be tested for interbreeding.
• Ring species and geographically isolated populations blur the boundary of a species.
• It is difficult to apply in captive or laboratory conditions.

Conclusion. The biological species concept is the most widely accepted and genetically meaningful definition of a species for sexually reproducing organisms. However, because of its limitations with asexual, fossil, and hybridising forms, taxonomists also use alternative concepts such as the morphological, ecological, phylogenetic, and evolutionary species concepts, often in combination.

Protozoa vs. Metazoa

Key point: Protozoa are single-celled organisms in which one cell is the whole body, while Metazoa are multicellular organisms in which cells are grouped into tissues and organs with division of labour.

Human intrusion in coral reefs

Coral reefs are among the most productive marine ecosystems, but human activities are damaging them seriously.

1. Overfishing and destructive fishing. Dynamite and cyanide fishing, trawling and removal of herbivorous fish let algae overgrow and smother corals.
2. Coastal development. Dredging, land reclamation, construction and mining of coral for building material destroy reef habitat directly.
3. Pollution. Sewage, fertiliser and pesticide run-off cause nutrient enrichment (eutrophication), algal blooms and disease. Oil spills and plastic waste also harm corals.
4. Sedimentation. Deforestation and soil erosion increase silt in coastal water, blocking light needed by the symbiotic zooxanthellae.
5. Tourism. Anchor damage, careless diving and snorkelling, trampling and souvenir collection break corals.
6. Climate change. Global warming raises sea temperature and causes coral bleaching, as corals expel zooxanthellae. Ocean acidification weakens the calcium carbonate skeleton.
7. Collection. Corals and reef fish are harvested for the aquarium trade and ornaments.

Consequences: loss of biodiversity, collapse of fisheries, loss of natural coastal protection against waves and storms, and loss of tourism income.

Conservation: marine protected areas, bans on destructive fishing, sewage treatment, sustainable tourism and reduction of greenhouse gas emissions.

Parasitic adaptations of Taenia solium (pork tapeworm)

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

1. Adaptations for attachment (to host intestinal wall)

• Scolex (head): bears a rostellum armed with two rows of chitinous hooks (22–32 hooks) and four muscular suckers. The suckers and hooks firmly anchor the worm to the intestinal mucosa so it is not washed away by peristalsis.
• Neck: short, undifferentiated region behind the scolex; it is the zone of growth from which new proglottids bud off.

2. Adaptations for protection

• Thick cuticle (tegument): a resistant, syncytial body covering that resists the host's digestive enzymes and immune attack.
• No digestive system: since it lives in a nutrient-rich medium, it absorbs pre-digested food directly through the general body surface (tegument), so a gut is unnecessary.
• Anaerobic respiration: lives in an oxygen-poor environment and respires anaerobically, so no respiratory or circulatory organs are needed.
• Antibody-destroying secretions: the tegument secretes substances that neutralise host antibodies, helping it evade the immune response.

3. Adaptations for reproduction (high fecundity)

• Hermaphrodite: each mature proglottid contains both male and female reproductive organs, so self-fertilisation is possible even if a single worm is present.
• Enormous egg output: a gravid proglottid contains about 30,000–50,000 eggs; the worm sheds several gravid proglottids daily, ensuring the survival of the species despite heavy losses.
• Strobila of many proglottids: the body is divided into a long chain of proglottids in different stages of maturity (immature, mature, gravid), so reproduction goes on continuously.

4. Adaptations for dispersal and transmission

• Oncosphere (hexacanth larva): the six-hooked embryo inside the egg is protected by a thick, resistant embryophore, so it survives in soil, water and vegetation until swallowed by the pig.
• Cysticercus (bladder worm): the larval stage in pig muscle is a fluid-filled bladder with an invaginated scolex; it survives in the host tissue and, when undercooked pork is eaten, the scolex evaginates and attaches to the human gut.
• Resistance to gastric juice: the egg shell and cyst wall withstand the acid and enzymes of the stomach, allowing the larva to reach the intestine unharmed.

5. Adaptations for a fixed, protected habitat

• The small intestine provides a warm, moist, nutrient-rich and sheltered environment; the worm's flattened, ribbon-like body offers a large surface area for absorption and lies closely applied to the mucosa.

Conclusion: The loss of digestive, respiratory and locomotory organs (degeneration) is compensated by the development of suckers, hooks, a resistant tegument, hermaphroditism and massive egg production — a classic example of degenerative and adaptive evolution in an endoparasite.

Structure and Sexual Dimorphism in Ascaris

Structure

Ascaris lumbricoides is a large, cylindrical, bilaterally symmetrical roundworm (Nematoda) that lives as an endoparasite in the small intestine of humans. It shows the typical nematode features:

• Body form: Elongated, cylindrical and tapering at both ends; the body is unsegmented and covered by a thick, resistant cuticle that protects the worm from the host's digestive enzymes.
• Body wall: Consists of an outer non-cellular cuticle, a syncytial epidermis (hypodermis), and a layer of longitudinal muscle cells. The muscle layer is only longitudinal, so the worm moves by whip-like, S-shaped bending movements.
• Body cavity: A pseudocoelom filled with pseudocoelomic fluid that acts as a hydrostatic skeleton and also distributes nutrients.
• Digestive system: A complete but simple tube — mouth → muscular pharynx → long non-muscular intestine → short rectum → anus. There is no circulatory or respiratory system; gaseous exchange occurs by diffusion through the body surface.
• Excretory system: Two longitudinal excretory canals joined by a transverse canal, opening to the outside through an excretory pore near the anterior end.
• Nervous system: A nerve ring around the oesophagus with dorsal and ventral longitudinal nerve cords, and sense organs in the form of papillae and amphids at the anterior end.
• Reproductive system: Sexes are separate (dioecious) and the worm shows marked sexual dimorphism.

Sexual Dimorphism

Males and females differ in the following characters:

Because the female is larger and produces a huge number of eggs, it is better adapted for the parasitic life cycle. The male is recognized at once by its ventrally curved tail bearing spicules.

Social Behaviour of Termites

Termites are social insects belonging to the order Isoptera. They live in highly organised colonies in which individuals are divided into different castes, each performing a specific function for the colony. This division of labour is the basis of their social behaviour.

1. Castes in a termite colony

A termite colony consists of three main castes:

• Reproductive caste (king and queen): The queen is the largest individual in the colony. She lays thousands of eggs and lives for many years. The king remains with the queen and fertilises her periodically. Besides the primary king and queen, there are supplementary reproductives that replace them if they die.
• Soldier caste: Soldiers are sterile, wingless individuals with large heads and strong mandibles. They defend the colony against ants and other enemies. Some soldiers (nasute type) eject a sticky secretion from the frontal gland instead of biting.
• Worker caste: Workers are the most numerous, sterile, and wingless. They do most of the work of the colony — building and repairing the nest, collecting food, feeding the queen and young ones, and caring for the eggs and larvae.

2. Colony formation (nuptial flight)

During the rainy season, winged reproductive males and females (alates) leave the nest in a swarming flight called the nuptial flight. They pair up, shed their wings, and the pair digs a small burrow in the soil. This pair becomes the king and queen of a new colony. The queen then begins to lay eggs, from which the sterile worker and soldier castes develop.

3. Division of labour

Each caste is specialised for a particular duty. Workers forage for cellulose-rich food such as wood, paper, and plant material. They chew the wood and feed the queen and soldiers by trophallaxis (mouth-to-mouth transfer of food). Soldiers only defend the colony and cannot feed themselves; they depend on workers for food. The queen only reproduces. Thus there is complete cooperation among the members.

4. Communication

Termites communicate mainly by pheromones (chemical signals). The queen secretes a pheromone that inhibits the development of new reproductives in the colony. Trail pheromones help workers find the path to food sources. Alarm pheromones released by soldiers warn the colony of danger. Antennal tapping and body vibrations also serve as signals.

5. Symbiotic association

Termites harbour flagellate protozoans (such as Trichonympha) in their hindgut. These protozoans secrete the enzyme cellulase that digests the cellulose of wood. In return, the termites provide food and shelter to the protozoans. This mutualism allows termites to utilise wood as food.

6. Nest building

Termites construct elaborate nests (termitaria) from soil, saliva, and faecal matter. Some species build tall mounds with a system of tunnels and chambers that maintain temperature and humidity. The nest has a royal chamber for the queen, brood chambers for eggs and larvae, and fungus gardens in some species.

7. Parental care and colony defence

The colony shows cooperative brood care — the eggs and young ones are looked after by the workers rather than by the parents alone. Soldiers guard the entrances and attack intruders. If a breach occurs, soldiers block the opening and workers quickly repair the wall. This self-sacrificing defence is a striking feature of termite social life.

Conclusion

Termites exhibit a well-developed social organisation with distinct castes, division of labour, cooperative brood care, chemical communication, and mutualistic association with gut protozoans. Because of these features, termites are regarded as truly social insects, comparable in organisation to ants and honeybees.

Molluscs (phylum Mollusca) form the second-largest animal phylum, with about 85,000 living species, and are of great importance in the following ways:

1. As food Many molluscs are highly nutritious and form a major part of the human diet, being rich in protein, calcium, iodine and vitamins. Edible forms include mussels, oysters, clams, squids (calamari) and octopus. Shellfish farming is a large industry in many coastal countries.

2. Source of ornaments and valuable products

• Pearls — obtained from pearl oysters and used in jewellery.
• Shells — used for making buttons, bangles, ornaments, lime and cement.
• Sepia — the ink of cuttlefish is used as a pigment and in medicines.
• Tyrian purple — a dye formerly obtained from certain sea snails.

3. Medicinal and industrial value Some molluscan extracts are used in traditional and modern medicine. Shell deposits (calcium carbonate) are used in the cement, lime and poultry-feed industries. Chitin from molluscan shells has industrial and biomedical uses.

4. Agricultural and economic importance

• Snails and slugs are pests that damage crops, vegetables and ornamental plants.
• Some snails serve as intermediate hosts of human and animal parasites — for example, the snail Bulinus transmits the blood fluke causing schistosomiasis, and Lymnaea transmits liver flukes. This makes their control important in public health.
• The shellfish trade earns valuable foreign currency for many nations.

5. Ecological importance Molluscs play a key role in the ecosystem:

• Bivalves (mussels, clams) are filter feeders and help purify water.
• They form an important link in food chains, being eaten by fish, birds and other animals.
• Burrowing forms aerate and mix the soil and sediment.
• Shells of dead molluscs add calcium carbonate to the soil and ocean floor.

6. Scientific importance Because of their soft body, shell and varied forms, molluscs are widely used in zoology practicals and research. The giant squid and octopus are studied for their advanced nervous system, and the squid giant axon has been used in nerve-impulse experiments.

Conclusion Molluscs are thus important as food, as sources of pearls, shells and medicines, as pests and disease carriers, and as essential members of aquatic ecosystems — making them economically, medically and ecologically significant.

Extra-cellular Digestion in Starfish (Asterias)

1. Feeding habit. The starfish is carnivorous and feeds mainly on bivalve molluscs such as mussels, oysters and clams. It has no jaws or teeth for chewing, so food is taken in large pieces and digested outside the body — hence the digestion is largely extra-cellular.

2. Capture of prey. The starfish crawls over the shell of a bivalve and attaches its tube feet to the two valves by suction. By a steady pull of the tube feet and arms, it tires the adductor muscle of the mollusc until the valves gape slightly.

3. Eversion of the stomach. The starfish then pushes its cardiac stomach out through the mouth by contraction of the body-wall muscles and the coelomic fluid pressure. The everted stomach is inserted between the gaping valves and comes into direct contact with the soft body of the prey. This is the key feature of extra-cellular digestion — the digestive surface is applied directly to the food outside the mouth.

4. Extra-cellular digestion. The cardiac stomach is richly supplied with pyloric caeca (digestive glands) which pour digestive juices containing proteolytic and other enzymes into it. These enzymes act on the tissues of the mollusc in the space between the valves, i.e. outside the starfish's body. The food is thus partly digested and converted into a semi-liquid mass (chyle) in the mantle cavity of the prey.

5. Absorption. The partly digested food is then drawn into the stomach and passed into the pyloric stomach and the pyloric caeca, where digestion is completed and absorption takes place. The pyloric caeca are the main sites of absorption and also store reserve food.

6. Egestion. The indigestible remains (shell fragments, etc.) are thrown out through the mouth when the cardiac stomach is withdrawn into the body.

Significance. This method allows the starfish to feed on prey much larger than its own mouth and to digest it while still enclosed in the shell — an adaptation to a carnivorous, slow-moving mode of life.

Torsion in gastropods

Torsion is a twisting of the visceral mass, mantle and mantle cavity through 180° anticlockwise (seen from above) relative to the head and foot. It occurs during the veliger larval stage and is a developmental event, not a trick of adult life.

Effects of torsion:

• The mantle cavity, with the gills, anus, nephridiopores and genital opening, shifts from the posterior to the anterior side, above the head.
• The nerve cords cross to form a figure-of-eight (streptoneury).
• The alimentary canal becomes U-shaped, and the originally paired organs of one side are reduced (often the right gill, auricle and kidney).
• The head can be withdrawn into the shell, with the foot closing it as an operculum.

Significance: the anterior mantle cavity lets clean water reach the gills and sense organs, and protects the delicate head. A disadvantage is that the anus and excretory openings discharge near the head, causing fouling.

Detorsion

Detorsion is the secondary reversal (partial or complete untwisting) of torsion, seen in Opisthobranchia (sea hares, sea slugs) and Pulmonata (land snails and slugs).

Effects of detorsion:

• The mantle cavity and its openings move back to the posterior or lateral side.
• The nerve cords untwist (euthyneury).
• The shell and gills are reduced or lost; in pulmonates the mantle cavity becomes a lung.
• The body becomes secondarily bilaterally symmetrical.

Herdmania has no trochophore larva; its larva is the tadpole (ascidian) larva. The trochophore is the larva of annelids and molluscs, so a description of Herdmania must be based on its tadpole larva.

Tadpole larva of Herdmania

It is a free-swimming, non-feeding larva about 1–2 mm long, with three regions:

• Head (trunk): oval, with an anterior adhesive papillae (three) for attachment. It contains the sensory vesicle with a statocyst and a pigmented ocellus, the brain, the pharynx with gill slits, and the digestive tract.
• Tail: long and muscular, with a notochord, a dorsal hollow nerve cord and bands of muscle, surrounded by a fin-like tunic.
• The larva shows all the three chordate characters: notochord, dorsal tubular nerve cord and pharyngeal gill slits.

Metamorphosis (retrogressive)

After a short free-swimming life (a few hours), the larva attaches by its adhesive papillae and is transformed into a sessile adult:

• The tail, with its notochord and nerve cord, is absorbed.
• The sensory vesicle and most of the nervous system degenerate, leaving a small ganglion.
• The pharynx enlarges with many gill slits, and the atrial cavity develops.

This loss of chordate characters is called retrogressive metamorphosis.

Diagram

Label in the diagram: adhesive papillae, sensory vesicle (statocyst and ocellus), pharynx with gill slits, notochord, dorsal nerve cord, muscle bands, tail fin, tunic.

Significance

The larva reveals the chordate nature of tunicates, which are not obvious in the sessile adult.

a. Trichomonas vaginalis A flagellate protozoan (phylum Protozoa) that lives in the human urogenital tract and causes trichomoniasis. It has four anterior flagella, one recurrent flagellum forming an undulating membrane, and an axostyle; it has no cyst stage and is transmitted sexually or through contaminated articles.

b. Sycon canal system The canal system of Sycon is of the syconoid type. Water enters through dermal ostia → incurrent canals → prosopyles → flagellated radial canals (choanocytes) → apopyles → spongocoel (paragaster) → osculum. It is more efficient than the asconoid type because the choanocyte-lined radial canals greatly increase the surface area for food collection.

c. Medusa of Obelia The medusa is the free-swimming, sexual, umbrella-shaped stage (genus Obelia). It bears a velum, a manubrium with a mouth at the centre, four radial canals and a ring canal, and four gonads on the radial canals. It is dioecious; gametes fuse to form a zygote that develops into the polyp colony, so the life cycle shows alternation of generations (metagenesis).

d. Filariasis A tropical disease caused by the nematode Wuchereria bancrofti (and Brugia spp.). The adult worms block lymphatic vessels, causing lymphoedema and elephantiasis. It is transmitted by Culex mosquitoes, in which the microfilariae develop into infective larvae.

e. Economic importance of Sitophilus oryzae The rice weevil is a serious stored-grain pest. Its grubs and adults bore into and feed on rice, wheat, maize and other cereals, reducing weight, quality and germination capacity and making the grain unfit for sale or seed. Control is by fumigation, drying of grain and storage in airtight containers.

f. Nephridia Excretory organs of annelids (e.g. earthworm). A nephridium is a coiled tube opening internally by a ciliated nephrostome into the coelom and externally by a nephridiopore. It removes nitrogenous waste (urea, ammonia) and excess water, thus maintaining water and salt balance. Three types occur: septal, integumentary and pharyngeal nephridia.

g. Mites and diseases Mites are small arachnids that act as vectors and pests. Sarcoptes scabiei causes scabies in humans; Dermatophagoides causes house-dust allergy and asthma; Dermanyssus and Ornithodoros transmit rickettsial diseases and relapsing fever; plant mites such as spider mites damage crops.

h. Statocyst of Unio A statocyst is the organ of balance (equilibrium) in Unio. It lies in the foot and consists of a sac lined with sensory cells and containing calcareous statoliths (statoconia). When the animal tilts, the statoliths press on the sensory hairs and the impulse is carried by nerves, helping the animal to maintain its normal position.

i. Pedicellariae Small, pincer-like appendages on the body surface of echinoderms (starfish, sea urchin). Each has a stalk and two or three jaws. They keep the body surface clean by removing debris and larvae, and also help in capturing prey and defence.

j. Coral bleaching The loss of colour of corals due to the expulsion of their symbiotic zooxanthellae (dinoflagellates) from the polyp tissues. It is caused mainly by rise in sea temperature, pollution and ocean acidification. Prolonged bleaching starves the coral and may kill the colony.