Trichomonas vaginalis — Life Cycle, Pathogenicity and Control Measures

1. Systematic Position

Trichomonas vaginalis is a flagellated protozoan parasite that causes trichomoniasis (a sexually transmitted disease) in humans. It lives in the urogenital tract of both males and females.

2. Life Cycle

T. vaginalis has a simple, monogenetic life cycle with only one stage — the trophozoite. There is no cystic stage.

Morphology of trophozoite:

• Pear-shaped (pyriform), measuring about 10–30 µm in length and 5–15 µm in width.
• Bears 4 anterior flagella and a 5th flagellum running backward along the surface, forming an undulating membrane.
• Has a prominent axostyle (supporting rod) projecting beyond the posterior end.
• Contains a single nucleus at the anterior end and a blepharoplast (kinetosome) at the base of flagella.
• Actively motile with a jerky, twitching movement.

Steps in the life cycle:

1. Habitat: The trophozoites live in the vagina, cervix, urethra, and prostate of infected persons.
2. Mode of infection: Transmission occurs mainly by sexual intercourse (direct contact). Rarely by indirect means such as contaminated toilet seats, towels, undergarments, and swimming pool water.
3. Multiplication: Inside the host, trophozoites multiply by longitudinal binary fission. The nucleus divides first (mitosis), followed by division of the cytoplasm and organelles.
4. No encystment: The parasite does not form cysts; it remains as a trophozoite throughout its life.
5. Survival outside host: Trophozoites can survive for a short time in moist environments (e.g., urine, semen, vaginal discharge) but are killed by drying, heat, and disinfectants.
6. New infection: When trophozoites reach a new host through sexual contact or contaminated articles, they colonize the urogenital mucosa and repeat the cycle.

Note: Because there is no cyst stage, the parasite is more fragile and cannot survive long outside the host. This is why direct sexual transmission is the main route.

3. Pathogenicity

A. In females (more common and severe):

• Vaginitis and urethritis: The parasite damages the vaginal epithelium, causing inflammation.
• Symptoms:
  • Profuse, frothy, yellowish-green vaginal discharge with a foul odor.
  • Vaginal itching, burning, and irritation.
  • Dyspareunia (pain during intercourse).
  • Dysuria (painful urination) and frequency of micturition.
  • Strawberry cervix (punctate hemorrhagic spots) on examination.
• Complications: May predispose to cervical cancer, pelvic inflammatory disease, and infertility. In pregnancy, it may cause premature rupture of membranes and low birth weight.

B. In males:

• Many males are asymptomatic carriers (about 50%).
• When symptomatic:
  • Urethritis with mild discharge.
  • Balanoposthitis (inflammation of glans and prepuce).
  • Prostatitis and epididymitis.
  • Dysuria and urethral itching.

C. Mechanism of pathogenesis:

• The trophozoite attaches to the urogenital mucosa via surface adhesins.
• It secretes cysteine proteinases and other cytolytic enzymes that damage host cells.
• It induces inflammatory response (neutrophil infiltration), causing tissue damage.
• It evades host immune response by antigenic variation and by coating itself with host proteins.

D. Complications in both sexes:

• Increased risk of HIV transmission and acquisition.
• Neonatal infection during passage through the birth canal (rare).

4. Control Measures

A. Preventive measures:

1. Sexual hygiene: Avoid multiple sexual partners; use condoms during intercourse.
2. Treatment of partners: Both sexual partners should be treated simultaneously to prevent reinfection.
3. Personal hygiene: Avoid sharing towels, undergarments, and toilet seats.
4. Avoid contact with contaminated water: Do not use public swimming pools if infected.
5. Health education: Awareness about STDs and safe sex practices.
6. Screening: Routine screening of pregnant women and high-risk groups.

B. Curative measures:

1. Drug of choice: Metronidazole (Flagyl) — oral, 2 g single dose or 500 mg twice daily for 7 days.
2. Alternative drugs: Tinidazole, Ornidazole, Secnidazole.
3. Topical treatment: Vaginal suppositories (e.g., metronidazole) may be used as adjunct.
4. Supportive care: Maintain local hygiene; treat secondary bacterial infections.
5. Follow-up: Repeat examination after treatment to confirm cure.

C. Public health measures:

• Notification and surveillance of cases.
• Contact tracing and treatment of sexual partners.
• Improved sanitation and health awareness programs.

5. Conclusion

Trichomonas vaginalis is a flagellated protozoan with a simple life cycle (trophozoite only, no cyst). It causes trichomoniasis, a common non-viral STD, producing vaginitis in females and urethritis in males. Control depends mainly on safe sexual practices, simultaneous treatment of partners, and metronidazole therapy.

Canal System in Porifera

Canal system (also called the water vascular system or aquiferous system) is the system of interconnecting spaces, canals and chambers through which water flows in the body of sponges. Water enters through ostia, passes through the spongocoel (paragaster), and exits through the osculum. It serves three main functions:

1. Nutrition — brings in food particles and oxygen, carries away waste.
2. Respiration — gaseous exchange by diffusion.
3. Excretion and reproduction — removal of metabolic wastes and distribution of sperms/eggs.

Types of Canal System

Sponges show three grades of canal system:

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Syconoid Type of Canal System

The syconoid type is found in sponges like Sycon (Scypha). It is an intermediate grade between the simple asconoid and the complex leuconoid type. The body wall is folded to form radial canals, which greatly increase the surface area for flagellated cells and thus improve the efficiency of water flow.

Structure

1. Dermal ostia (incurrent pores): Numerous small pores on the outer surface (pinacoderm) through which water enters from outside.

2. Incurrent canals: Short canals leading inward from the ostia. These are lined by flattened pinacocytes (not choanocytes).

3. Prosopyles: Small openings at the inner ends of incurrent canals through which water passes into the radial canals. These are guarded by contractile cells (myocytes).

4. Radial canals (flagellated canals): These are the characteristic feature of the syconoid type. They are vertical, finger-like outfoldings of the body wall arranged radially around the central spongocoel. They are lined by choanocytes (flagellated collar cells) which create the water current by beating their flagella. The radial canals open into the spongocoel through apopyles.

5. Spongocoel (paragaster): The central body cavity lined by flattened pinacocytes. It receives water from all radial canals.

6. Osculum: The single large opening at the top through which water leaves the body.

Path of Water Flow

$\text{Outside} \rightarrow \text{Dermal ostia} \rightarrow \text{Incurrent canals} \rightarrow \text{Prosopyles} \rightarrow \text{Radial canals (choanocytes)} \rightarrow \text{Apopyles} \rightarrow \text{Spongocoel} \rightarrow \text{Osculum} \rightarrow \text{Outside}$

Diagram

                    OSCULUM
                       |
        ___________    |    ___________
       /           \   |   /           \
      /  spongocoel  \ | /  spongocoel  \
     |   (paragaster)  |  (paragaster)   |
     |                 |                 |
     |   [apopyles]    |    [apopyles]   |
     |    |  |  |      |     |  |  |     |
     |  __|__|__|__    |   __|__|__|__   |
     | |  radial   |   |  |  radial   |  |
     | |  canals   |   |  |  canals   |  |
     | | (choano-  |   |  | (choano-  |  |
     | |  cytes)   |   |  |  cytes)   |  |
     | |___________|   |  |___________|  |
     |   [prosopyles]   |   [prosopyles]  |
     |    |  |  |       |    |  |  |      |
     |  incurrent      |  incurrent      |
     |   canals         |   canals        |
     |  [dermal ostia]  |  [dermal ostia] |
     |___|__|__|________|________|__|__|__|
              outer surface (pinacoderm)

Key Features of Syconoid Type

• The choanocytes are restricted to the radial canals only, not the spongocoel.
• The body wall is folded, increasing surface area for choanocytes.
• Water flow is more efficient than in asconoid type because of the increased flagellated surface.
• The spongocoel is not lined by choanocytes but by pinacocytes.
• It represents a transitional stage between asconoid and leuconoid types.

Significance

The syconoid canal system increases the efficiency of the sponge by:

• Increasing the surface area available for food capture and gas exchange.
• Creating a more directed and forceful water current.
• Allowing the sponge to grow larger than asconoid forms, since the spongocoel is no longer the only site of choanocytes.

Vermicomposting

Vermicomposting is the process of converting biodegradable organic waste (kitchen waste, crop residues, animal dung, etc.) into a nutrient-rich, humus-like material called vermicompost (also called vermicast or worm castings) through the digestive action of earthworms, mainly Eisenia fetida (red wiggler) and Eudrilus eugeniae. The earthworms ingest the organic matter, and the material that passes through their gut is broken down by enzymes and microbes, producing a dark, odourless, granular end product that is an excellent organic fertilizer.

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Preparation of Vermicompost

1. Selection of site A cool, shaded place, protected from direct sunlight and rain, is chosen. A thatched shed or tree shade is ideal, since earthworms cannot tolerate high temperature and light.

2. Construction of the bed / pit

• Bed method: A bed of size 2 m × 1 m × 0.5 m is prepared on a hard surface using bricks or stones, or a polythene sheet is spread to prevent worms from escaping into the soil.
• Pit method: A pit of about 2 m × 1 m × 0.6 m is dug in a shaded area.
• A layer of sand or loamy soil (5–10 cm) is spread at the bottom to retain moisture.

2. Collection and partial decomposition of waste Biodegradable waste such as vegetable peels, leaves, grass, straw, and cattle dung is collected. Non-biodegradable materials (plastics, glass, metals) and oily or spicy food are avoided. The waste is heaped and kept for 15–20 days to allow partial decomposition (pre-digestion), which makes it soft and suitable for the worms. It is turned 2–3 times during this period.

3. Introduction of earthworms The partially decomposed waste is spread in layers of about 15–20 cm in the bed. Earthworms are released at the rate of about 1 kg per square metre (roughly 1000 worms per bed). Cow dung slurry may be sprinkled to keep the bed moist.

4. Maintenance

• The bed is kept moist (about 40–50% moisture) by sprinkling water at intervals; waterlogging must be avoided.
• The bed is covered with dry leaves, straw or jute bags to maintain moisture and darkness.
• Temperature is maintained at about 20–30 °C.
• The bed is turned gently every 7–10 days to aerate it and mix the material uniformly.
• Fresh waste can be added in thin layers on top from time to time.

5. Harvesting In about 45–60 days, the material turns dark brown, granular, and odourless, indicating that the compost is ready. 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.

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Significance of Vermicompost

1. Rich in nutrients: It contains nitrogen (1–2%), phosphorus (0.5–1%), potassium (0.5–1%), besides calcium, magnesium, sulphur, and micronutrients such as iron, zinc, copper and manganese in available forms.

2. Improves soil fertility and structure: It increases organic matter, improves soil porosity, aeration, water-holding capacity, and makes the soil loose and friable.

3. Enhances microbial activity: It is rich in beneficial bacteria, fungi, and actinomycetes, which help in nutrient cycling and suppress soil-borne pathogens.

4. Promotes plant growth: Growth-promoting substances such as auxins, gibberellins, and cytokinins present in vermicompost stimulate seed germination, root development, and overall plant growth.

5. Eco-friendly waste management: It converts biodegradable waste into a useful product, reducing the burden on landfills and preventing pollution and greenhouse gas emission.

6. Reduces dependence on chemical fertilizers: Its use lowers the cost of chemical fertilizers and reduces their harmful effects on soil, water, and human health.

7. Sustainable and low-cost: It can be prepared at home or farm level using locally available waste and earthworms, making it suitable for organic farming.

8. Improves crop quality and yield: Crops grown with vermicompost show better yield, taste, and keeping quality, with reduced pest and disease incidence.

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Conclusion

Vermicomposting is a simple, cheap, and environment-friendly technology that converts organic waste into a valuable organic fertilizer. It improves soil health, sustains crop productivity, and helps in the safe disposal of biodegradable waste, and is therefore an important component of sustainable agriculture.

Life History and Economic Importance of Aedes aegypti

Systematic position Phylum: Arthropoda; Class: Insecta; Order: Diptera; Family: Culicidae; Genus: Aedes; Species: aegypti.

Habit and habitat Aedes aegypti is a domestic, day-biting mosquito closely associated with human dwellings. It breeds in small collections of clean, stagnant water such as earthen pots, water tanks, tyres, tin cans, flower vases and drains. The adults are active mainly during the early morning and late afternoon, and the female is the blood-sucking stage.

Life history (complete metamorphosis) The life cycle is holometabolous and passes through four stages: egg, larva, pupa and adult. It is completed in about 8–12 days under favourable conditions.

1. Egg – The female lays 100–300 eggs singly on the inner wall of water containers, just above the water line. The eggs are black, oval and about 0.5 mm long. They are resistant to desiccation and can survive for several months in dry conditions. Hatching occurs within 2–3 days when the eggs come in contact with water.

2. Larva – The larva is aquatic, worm-like and commonly called a "wriggler". It has a distinct head, thorax and abdomen, and hangs at the water surface with the help of a siphon to breathe atmospheric air. It feeds on organic matter, bacteria and algae. It moults four times and grows through four larval instars in about 5–7 days.

3. Pupa – The pupa is aquatic, comma-shaped and called a "tumbler" because it tumbles actively when disturbed. It does not feed but breathes through two respiratory trumpets at the water surface. The pupal stage lasts about 2–3 days.

4. Adult – The pupa develops into an adult within the pupal case, which splits at the water surface. The adult emerges, rests on the water surface until its wings dry, and then flies away. The female feeds on human blood (needed for egg maturation), while the male feeds on plant juices and nectar. The adult life span is about 2–4 weeks. The female may lay eggs several times in her life.

Economic importance Aedes aegypti is of great economic importance mainly as a vector of several serious human diseases.

1. Dengue fever and dengue haemorrhagic fever – It is the principal vector of the dengue virus. Dengue is a major public health problem in Nepal and other tropical countries, causing high morbidity, hospitalisation and death.

2. Chikungunya – The virus is transmitted by the bite of infected Aedes aegypti, causing fever and severe joint pain.

3. Zika virus disease – It is also transmitted by this mosquito and is associated with neurological complications and birth defects.

4. Yellow fever – In Africa and South America, Aedes aegypti is the main urban vector of yellow fever virus.

5. Other effects – Mosquito bites cause irritation, allergic reactions and loss of work efficiency. The diseases it spreads lead to heavy expenditure on treatment, loss of working days, reduced productivity and, in severe cases, death. Thus it imposes a great economic burden on individuals and the nation.

Control measures Elimination of breeding sites by emptying water containers, use of larvicides and insecticides, screening of houses, use of mosquito nets and repellents, and biological control by introducing larvivorous fish are the main control measures.

Respiratory Organs and Mode of Respiration in Pila globosa

Introduction

Pila globosa (the apple snail) is a freshwater gastropod found in ponds, tanks and paddy fields. It is aquatic but air-breathing, so it must come to the surface periodically to take in atmospheric air. It possesses two sets of respiratory organs — one for aquatic (water) respiration and one for aerial (air) respiration. This dual arrangement is called bimodal respiration.

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Respiratory Organs

1. Ctenidium (Gill) — for aquatic respiration

• A single, well-developed comb-like gill (ctenidium) lies in the mantle cavity on the left side of the body.
• It is the primary aquatic respiratory organ and is used mainly by young snails and by adults when submerged in well-aerated water.
• Structure: the ctenidium has a central axis from which two rows of triangular gill filaments (lamellae) project, giving it a comb-like (pectinate) appearance.
• Each filament contains:
  • Afferent and efferent blood vessels (carrying deoxygenated and oxygenated blood respectively),
  • A rich capillary network for gaseous exchange,
  • Chitinous supporting rods that keep the filaments stiff and prevent collapse.
• Water enters the mantle cavity through the inhalant siphon, passes over the gill filaments, and leaves through the exhalant siphon. Dissolved $O_2$ diffuses from water into the blood; $CO_2$ diffuses out.

2. Pulmonary Sac (Lung) — for aerial respiration

• The pulmonary sac is the chief respiratory organ of the adult and is used for breathing atmospheric air.
• It is a highly vascularised, bag-like diverticulum of the mantle cavity, located on the right side of the body.
• Its inner wall is folded and richly supplied with blood capillaries, giving it a spongy, lung-like appearance.
• It opens to the exterior through a pneumostome (a slit-like opening) situated at the anterior right margin of the mantle collar.
• The pulmonary sac is not a true lung (it is not derived from the gut), but functionally it acts as one — hence the name "lung sac."

3. Accessory respiratory surfaces

• The moist skin of the body and the mantle also help in cutaneous gaseous exchange to a small extent.
• In young individuals, the ctenidium is more active; in adults, the pulmonary sac takes over as the main organ.

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Mode of Respiration

A. Aquatic (branchial) respiration

1. Water containing dissolved oxygen enters the mantle cavity through the inhalant siphon.
2. It flows over the ctenidium, where dissolved $O_2$ diffuses into the blood capillaries and $CO_2$ diffuses out into the water.
3. Water then passes out through the exhalant siphon.
4. This mode is used when the snail is submerged in water.

B. Aerial (pulmonary) respiration

1. The snail rises to the water surface and pushes the pneumostome above the water film.
2. The floor of the mantle cavity (the diaphragm) contracts and relaxes, creating a pumping action that draws atmospheric air into the pulmonary sac.
3. Inside the sac, $O_2$ diffuses into the blood capillaries and $CO_2$ diffuses out into the air.
4. The pneumostome then closes, and the snail may remain submerged for some time using the stored air.
5. When the air becomes stale, the snail again comes to the surface and renews it.

C. Cutaneous respiration

• The thin, moist, vascular skin and mantle surface absorb oxygen directly from water or air, supplementing the above two modes.

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Significance of Bimodal Respiration

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Conclusion

Pila globosa is a bimodal breather. It possesses a ctenidium for aquatic respiration and a pulmonary sac for aerial respiration, supported by cutaneous respiration. The ctenidium is the main organ in young snails and in submerged conditions, while the pulmonary sac becomes the chief organ in adults, especially during aestivation and in oxygen-deficient water. This dual respiratory arrangement is a perfect adaptive feature for its freshwater, amphibious mode of life.

Affinities of Amphioxus (Branchiostoma)

Amphioxus (genus Branchiostoma) is a small, fish-like, marine chordate that lives buried in sand in shallow seas. Because it shows the three fundamental chordate characters throughout life but lacks a true head and vertebral column, it is regarded as a typical primitive chordate and is placed in the subphylum Cephalochordata. Its affinities are discussed below.

1. Affinities with Chordata (Chordate characters)

Amphioxus exhibits all three diagnostic chordate features:

• Notochord: A dorsal, elastic, rod-like notochord extends from anterior to posterior end and persists throughout life.
• Dorsal hollow nerve cord: A tubular nerve cord lies dorsal to the notochord; its anterior end is slightly enlarged but no brain is formed.
• Pharyngeal gill slits: Numerous paired gill slits open from the pharynx to the exterior and are used for filter feeding and respiration.

It also shows other chordate features: a post-anal tail, a closed blood system without a heart (blood pumped by contractile vessels), a coelom, and metameric (segmented) muscle blocks called myomeres. Hence its chordate affinity is unquestionable.

2. Affinities with Vertebrata

Amphioxus resembles vertebrates in several ways:

• Presence of a notochord, dorsal hollow nerve cord and pharyngeal gill slits.
• A segmented body with myomeres and a segmentally arranged nerve cord with dorsal and ventral roots.
• A hepatic caecum comparable to the vertebrate liver, and a comparable excretory system (protonephridia with solenocytes).
• Closed circulatory system and a post-anal tail.

However, it differs from vertebrates in lacking a cranium, brain, vertebral column, paired fins, jaws, a muscular heart, a true liver, and a well-developed kidney. Therefore it is regarded as a primitive chordate rather than a vertebrate, and is often called a "living fossil" showing the ancestral vertebrate plan.

3. Affinities with Hemichordata

It shows some resemblance to hemichordates (e.g. Balanoglossus): both are marine, have gill slits, a coelom, and a comparable mode of development. But Amphioxus has a true notochord and a dorsal hollow nerve cord, which hemichordates lack. Hence it is placed higher than Hemichordata.

4. Affinities with Urochordata

With urochordates (e.g. Ascidia) it shares the notochord, dorsal hollow nerve cord and gill slits, but in urochordates these are restricted to the larval stage and the adult is degenerate and sessile. In Amphioxus they persist throughout life, so it is considered more advanced than Urochordata.

5. Affinities with Annelida (invertebrate resemblance)

Superficially it resembles annelids in being segmented, having a coelom, and in the segmental arrangement of muscles, nerves and gonads. But the segmentation is different in nature, and the presence of a notochord, dorsal hollow nerve cord and gill slits clearly separates it from annelids. Such resemblances are only superficial.

6. Affinities with Mollusca

It shows a slight resemblance to molluscs in having a ciliated wheel organ (comparable to the velum of molluscan larvae) and a hepatic caecum. These are, however, only analogous features and do not indicate real relationship.

Conclusion

Amphioxus is a primitive chordate that stands at the base of the chordate line. It combines chordate characters with certain invertebrate-like features, and its study is important because it throws light on the origin of vertebrates. It is therefore rightly called a "living fossil" and is placed in the subphylum Cephalochordata of the phylum Chordata.

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 calipers or rulers — for example body length, head width, wing length, tail length, or ratios such as tail/body length. Data are analysed by mean, standard deviation, coefficient of variation, t-test, and simple or multiple regression. It is simple and widely used, but treats each character independently and cannot capture shape as a whole.

2. Multivariate morphometrics. Many correlated characters are measured on each specimen and analysed together using principal component analysis (PCA), discriminant function analysis (DFA), canonical variate analysis (CVA), and cluster analysis. PCA reduces many correlated variables to a few independent components; DFA and CVA find the combination of characters that best separates known groups and allow classification of unknown specimens. This is powerful for separating closely related species.

3. Geometric morphometrics. Landmarks (biologically homologous points) and outlines are digitised from images or 3D scans. Shape is extracted by removing size, position, and rotation (Procrustes superimposition), and analysed by thin-plate spline, relative warp analysis, and partial least squares. It preserves the geometry of the organism, visualises shape change as deformation grids, and quantifies variation that traditional methods miss.

Applications in taxonomy.

• Discriminating sibling and cryptic species where external differences are subtle.
• Determining the status of populations (whether they are distinct species, subspecies, or ecotypes).
• Studying sexual dimorphism, geographic variation (clines), and ontogenetic (growth) changes.
• Reconstructing phylogenies and supporting molecular data with phenotypic evidence.
• Identifying specimens in applied fields such as pest and vector identification.

Limitations. Results depend on sample size and on the choice of characters and landmarks; measurements may be affected by age, sex, nutrition, and environment (phenotypic plasticity); and shape differences alone cannot establish reproductive isolation. Hence morphometrics is best used together with molecular, ecological, and behavioural data.

Conclusion. Morphometric methods convert morphology into quantitative data suitable for statistical testing. Traditional, multivariate, and geometric approaches together provide a rigorous, objective basis for species delimitation and identification in modern taxonomy.

Conjugation in Vorticella

Vorticella is a stalked, sessile peritrich ciliate. Under unfavourable conditions it reproduces sexually by conjugation, which is temporary (the partners separate afterwards) and anisogamous (the two partners are unequal).

Process

1. A vegetative cell divides repeatedly by unequal binary fission, without growth, into small cells. These lose the stalk and peristome and become free-swimming microconjugants (microgametes). An ordinary stalked cell acts as the macroconjugant.
2. A microconjugant attaches to a macroconjugant and a conjugation bridge forms between them.
3. The micronucleus of each partner divides by meiosis into four haploid nuclei; three degenerate and one divides mitotically into two pronuclei, one stationary and one migratory.
4. The migratory pronucleus of the microconjugant passes through the bridge and fuses with the stationary pronucleus of the macroconjugant (karyogamy) to form a diploid synkaryon.
5. The old macronucleus degenerates. The synkaryon divides repeatedly to form the new macronucleus and micronucleus.
6. The microconjugant, now empty, degenerates, and the macroconjugant (exconjugant) resumes binary fission.

Significance: genetic recombination, rejuvenation of the clone, and a means of surviving unfavourable conditions.

Corals and Coral Reefs

Corals are marine invertebrates belonging to the phylum Cnidaria (class Anthozoa). They exist as polyps — small, cylindrical animals with a mouth surrounded by tentacles — which secrete a hard external skeleton of calcium carbonate (CaCO₃). Most reef-building corals are colonial, living in large groups where each polyp contributes to a common limestone framework. Most corals also contain zooxanthellae, symbiotic photosynthetic algae that supply them with nutrients and give them colour.

Coral reefs are large underwater structures built up over thousands of years from the accumulated CaCO₃ skeletons of corals and other organisms. They are the most diverse marine ecosystems on Earth.

Types of reefs:

1. Fringing reefs — grow close to the shore, directly bordering land.
2. Barrier reefs — separated from the shore by a lagoon (e.g., Great Barrier Reef).
3. Atolls — ring-shaped reefs enclosing a central lagoon, often formed on sunken volcanic islands.

Conditions for growth: warm water (20–28 °C), shallow depth (up to ~50 m) for sunlight, clear sediment-free water, and normal salinity.

Importance:

• Act as natural barriers, protecting coastlines from erosion and storm waves.
• Provide habitat and breeding grounds for a huge diversity of fish and marine life.
• Support fisheries and tourism, contributing to the economy.
• Source of medicines and building materials (lime).

Threats: coral bleaching due to rising sea temperature, ocean acidification, pollution, sedimentation, destructive fishing, and overexploitation. Conservation requires marine protected areas, reduced pollution, and control of global warming.

Amoebiasis (Amoebic Dysentery)

Amoebiasis is a protozoan disease of the human large intestine caused by Entamoeba histolytica. It is transmitted by the faecal–oral route through contaminated food and water, and is common in areas with poor sanitation.

Causative agent: Entamoeba histolytica (trophozoite is the active, invasive form; cyst is the infective form).

Mode of infection: Mature quadrinucleate cysts are swallowed with contaminated food, water, or from soiled hands. Excystation occurs in the small intestine, releasing trophozoites that colonise the colon. Cysts passed in faeces of carriers contaminate water and food.

Symptoms

1. Intestinal (amoebic dysentery):
   • Intermittent diarrhoea alternating with constipation
   • Blood and mucus in stool (characteristic "anchovy sauce" stool)
   • Abdominal pain and cramps, especially in the right lower abdomen
   • Tenesmus (painful urge to defecate) and excessive gas
   • 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, jaundice
   • Amoebic hepatitis — enlarged, tender liver
   • Rarely, lung and brain abscesses
3. Asymptomatic carriers — pass cysts without symptoms and act as sources of infection.

Control Measures

1. Sanitation: Proper disposal of human excreta; use of sanitary latrines; protection of water sources from sewage.
2. Safe water and food: Drink boiled or chlorinated water; wash vegetables and fruits in clean water; avoid uncovered and stale food; protect food from flies and cockroaches.
3. Personal hygiene: Wash hands with soap before eating and after using the toilet; keep nails short; avoid defecating in open fields.
4. Screening and treatment: Detect and treat patients and carriers; drugs such as metronidazole (with a luminal amoebicide like diloxanide furoate or paromomycin to clear cysts).
5. Health education: Educate the public about the mode of transmission, personal cleanliness, and the importance of safe drinking water.
6. Fly and vector control: Use insecticides and cover food to prevent mechanical transmission by flies.
7. Isolation of patients: Avoid handling food while infected; disinfect stools and soiled linen properly.

Conclusion: Amoebiasis is a preventable disease; improvement in environmental sanitation, safe water supply, and personal hygiene, together with early diagnosis and treatment, can effectively control it.

Copulation and Cocoon Formation in Leech

Leeches (e.g. Hirudinaria, Hirudo) are hermaphrodite but cross-fertilising.

Copulation

• Two leeches come together with their ventral surfaces touching, head to tail, so that the clitellar region of each lies against the genital region of the other.
• The male gonopore of each (segment 10) deposits spermatophores on the partner's clitellar region. The sperm are released, pass through the body wall (or are introduced near the female gonopore), and are stored in the female reproductive tract.
• Fertilisation is internal. After copulation the leeches separate.

Cocoon formation

1. The clitellum (segments 9-11) secretes a mucous ring and, beneath it, an albuminous fluid that surrounds the body as a sheath.
2. The leech withdraws its head backward out of the ring. As the ring passes the female gonopore, fertilised eggs and albumen are passed into it.
3. The ring slips off over the head, and its two ends close and harden into a lemon-shaped cocoon.
4. The cocoon is deposited in moist soil or on aquatic vegetation, where the young leeches develop directly (no larva) and hatch as small leeches.

Metamorphosis in Insects

Metamorphosis is the series of structural, physiological and behavioural changes by which an insect passes from the newly hatched young stage to the adult. It is controlled by hormones, mainly ecdysone and juvenile hormone.

Types

1. Ametabolous (no metamorphosis) The young hatches looking like a small adult, lacking only full size and mature genitalia, and grows by moulting. Example: silverfish (Lepisma).

2. Hemimetabolous (incomplete metamorphosis) Egg → nymph → adult. The nymph resembles the adult but has no functional wings or reproductive organs; wings develop gradually as external buds (exopterygote). Examples: grasshopper, cockroach, bug, termite. When the nymph is aquatic and differs from the adult (called a naiad), as in dragonflies and mayflies, the change is more marked.

3. Holometabolous (complete metamorphosis) Egg → larva → pupa → adult. The larva (caterpillar, grub, maggot) differs completely from the adult in form, food and habitat. The pupa is a resting stage in which larval tissues break down and adult organs form (endopterygote). Examples: butterfly, housefly, mosquito, beetle, honeybee.

4. Hypermetamorphosis A special form of complete metamorphosis in which the larva passes through several different forms before pupation. Example: blister beetle (Meloe), whose first-stage larva (triungulin) differs from the later grub-like stages.

Significance: the larva and adult use different food and habitats, which reduces competition between them.

Torsion and Detorsion in Molluscs

Torsion

Torsion is a twisting of the visceral mass and mantle through 180° relative to the head and foot. It is characteristic of gastropods and occurs during the veliger larval stage, within a few minutes to hours. It is caused by unequal contraction of the larval retractor muscles (the right one is stronger).

Effects of torsion

• The mantle cavity, gills (ctenidia), anus and excretory and genital openings shift from the posterior to the anterior side, above the head.
• The gills and auricles of the two sides exchange positions; the original left gill lies on the right.
• The alimentary canal becomes U-shaped and the visceral nerve loop is twisted into a figure of eight (streptoneury).

Significance

• The head can be withdrawn into the shell first and the mantle cavity gives protection.
• Clean water enters the mantle cavity from the front, near the sense organs.
• Disadvantage: the anus and nephridiopores open near the head and gills, so faeces and waste may foul the water entering the mantle cavity. Many gastropods reduce this by losing the right gill or by a slit or hole in the shell.

Detorsion

Detorsion is the partial or complete reversal (untwisting) of torsion. It is seen in opisthobranchs (e.g. sea hares, nudibranchs) and some pulmonates. The mantle cavity and anus return to a posterior or lateral position, the nerve loop is untwisted (euthyneury), the shell is reduced or lost and the original gill may be replaced by secondary gills (cerata).

Tube Feet of Star Fish

Tube feet (podia) are the characteristic locomotory and adhesive organs of echinoderms, and in the star fish (Asterias) they form the main effector system of the water-vascular system. They are numerous, small, soft, hollow, finger-like extensions borne on the oral (lower) surface of each arm, arranged in the ambulacral grooves.

Structure

• Each tube foot is a thin-walled, cylindrical or conical tube, closed and usually sucker-like (with a terminal disc) at its free end.
• Its wall is a continuation of the body wall and consists of an outer thin cuticle, an epidermis (ciliated columnar epithelium), a layer of connective tissue, and an inner layer of circular muscles and longitudinal muscle fibres (the longitudinal fibres are better developed and act as retractors).
• The base of the tube foot communicates with a small muscular sac, the ampulla, which lies above it inside the arm and opens into the radial canal through a short lateral canal guarded by a valve.
• The cavity of the tube foot and ampulla is filled with coelomic fluid (water-vascular fluid) and is thus a closed hydraulic unit.

Working

1. Extension (protrusion): Contraction of the ampulla (its muscles squeeze the fluid) forces water into the tube foot; the valve prevents backflow. Since the fluid is incompressible, the tube foot is elongated and pressed outward through the ambulacral groove.
2. Attachment: The terminal sucker is pressed against the substratum; secretion and suction make it adhere firmly.
3. Anchoring: Contraction of the longitudinal (retractor) muscles of the tube foot shortens it and pulls the body forward, while the sucker holds fast.
4. Release: Relaxation of the ampulla and contraction of the muscles allow fluid to return to the ampulla, the sucker loosens, and the foot is withdrawn.

The tube feet of the two sides of an arm and of different arms act in a coordinated wave (metachronal rhythm), so the animal creeps slowly in any direction.

Functions

• Locomotion: Chief organs of slow creeping movement over rocks and sand.
• Adhesion and grip: Hold the animal firmly to the substratum and to prey; used to pull open bivalve shells during feeding.
• Respiration: Thin, highly vascular walls serve as accessory respiratory surfaces for gaseous exchange.
• Food handling and sensation: Help in capturing and manipulating food and in tactile (sensory) perception.
• Excretion: Nitrogenous wastes diffuse out through their thin walls.

Conclusion

The tube feet are thus hydraulic, muscular, sucker-bearing organs of the water-vascular system that serve mainly for locomotion and adhesion, and secondarily for respiration, feeding and excretion — a unique feature of the phylum Echinodermata.

Life Cycle of Ascaris lumbricoides

Ascaris lumbricoides is the common roundworm of humans. The life cycle is direct and monogenetic: only one host (man) is needed.

Adults and eggs

• The adults are dioecious and live in the small intestine. A female lays about 2,00,000 eggs per day.
• The eggs are passed in faeces. Fertilised eggs have a thick, mamillated coat; unfertilised eggs are not infective.

Development outside the host

• In warm, moist soil the fertilised egg develops over about 2 to 3 weeks. The first-stage larva forms inside the shell and moults to the second-stage larva, which is the infective stage and stays inside the egg.

Infection

• Man is infected by swallowing embryonated eggs with contaminated food, water or soiled hands (faecal-oral route), a common problem where sanitation is poor.

Migration in the body

1. The eggs hatch in the small intestine and the larvae penetrate the intestinal wall.
2. Through the portal vein they reach the liver, then the right heart and the lungs.
3. In the lungs they moult twice (to the fourth stage) and break into the alveoli.
4. They climb the bronchi and trachea, are coughed up and swallowed, and return to the small intestine.
5. There they moult a final time and mature into adults in about 2 to 2.5 months.

Effects: abdominal pain, malnutrition in children, pneumonitis during lung migration, and intestinal obstruction in heavy infection. Prevention: handwashing, sanitation and deworming.

Silk of Moth and Its Uses

Silk is a natural protein fibre produced by the larvae (caterpillars) of certain moths, chiefly the silkworm Bombyx mori (family Bombycidae). It is the most important animal fibre after wool and is valued for its lustre, smoothness, lightness and strength.

Source and formation of silk

• The silk is secreted by the silk glands (labial glands) of the caterpillar, which are modified salivary glands lying along the sides of the body and opening on the spinneret (a small papilla on the lower lip).
• The secretion consists of a protein called fibroin, cemented together by another protein called sericin (silk gum). It is extruded through the spinneret as a fine continuous filament.
• When the larva is fully grown (about the fifth instar), it spins the filament around itself and forms a cocoon for pupation. A single cocoon may contain a filament 300–900 metres long.
• Inside the cocoon the larva changes into a pupa, which later becomes the adult moth. The adult moth emerges by secreting an alkaline fluid that softens one end of the cocoon.

Types of silk

1. Mulberry silk — obtained from Bombyx mori, which feeds on mulberry leaves; the finest and most commercial silk.
2. Non-mulberry (wild) silks — 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)

Rearing of silkworms is called sericulture. The cocoons are collected and stifled (killed by hot water, steam or hot air) to prevent the moth from breaking the filament. The cocoons are then reeled to unwind the silk thread, and the gum (sericin) is removed by degumming so that the fibre becomes soft and glossy. The raw silk is then spun, dyed and woven.

Uses of silk

1. Textiles — sarees, shawls, scarves, ties, shirts, dresses and other luxury garments; also blended with cotton, wool or synthetic fibres.
2. Household goods — curtains, upholstery, cushion covers, bed sheets and wall hangings.
3. Medical and surgical use — silk sutures (stitches) for wound closure, as silk is strong, non-toxic and sterilizable; also used in some dressings.
4. Technical and industrial use — insulating material in electrical equipment, parachutes, tyre cords, fishing lines and sieves.
5. Other uses — silk waste and pierced cocoons are spun into spun silk; pupae are used as poultry and fish feed; silk fibre is used in making artificial blood vessels and in cosmetics.
6. Economic importance — sericulture provides employment and income to farmers and weavers, and silk is an important export commodity (especially in China, India and Nepal).

Conclusion

Silk is a proteinaceous secretion of the silkworm moth, spun into a cocoon and reeled into a valuable textile fibre. Because of its unique combination of fineness, strength, lustre and biocompatibility, silk finds wide use in clothing, medicine, industry and the rural economy.

a. Nephridia Excretory organs of annelids (e.g., earthworm). They are coiled, ciliated tubules that collect nitrogenous waste from the coelom and blood and discharge it to the exterior through nephridiopores. Three types occur: septal, integumentary and pharyngeal nephridia.

b. Calcareous spicule and significance Calcareous spicules are small calcareous skeletal elements secreted by sclerocytes in sponges (e.g., Sycon). Significance: they form the skeleton, give support and rigidity to the body wall, keep the canal system open, and protect the sponge from predators.

c. Dactylozooids in coelenterate Dactylozooids are defensive zooids of colonial coelenterates such as Physalia (Portuguese man-of-war). They are finger-like, bear numerous nematocysts, and have no mouth or tentacles; their function is to capture prey and defend the colony.

d. Sexual dimorphism in Ascaris The male and female differ in size and structure. Female: longer (20–35 cm), straight or slightly curved, posterior end pointed and straight, with a separate gonopore. Male: shorter (15–30 cm), posterior end curved and hooked, with a cloaca and two penial setae; hence they are sexually dimorphic.

e. Parasitism A mode of heterotrophic nutrition in which one organism (parasite) lives on or inside another organism (host), obtains food from it and harms it, but does not kill it immediately. Example: Ascaris in the human intestine, Taenia in the human gut.

f. Jaws of leech The leech has three muscular jaws (one dorsal and two ventrolateral) lying in the buccal cavity. Each jaw bears a row of about 60–100 minute, sharp, chitinous teeth. The jaws make a Y-shaped bite on the host's skin to suck blood.

g. Statocysts of prawn Statocysts are the balancing (equilibrium) organs of prawn, located in the basal segment of each antennule. Each is a sac lined with sensory setae and containing statoliths (sand grains). They help the animal maintain its normal posture and balance in water.

h. Dentalium Dentalium is a marine mollusc of the class Scaphopoda, commonly called the tusk shell. Its shell is tubular, open at both ends, and curved like an elephant's tusk. It burrows in sand and feeds on microscopic organisms by means of tentacles.

i. Retrogressive metamorphosis A type of metamorphosis in which the larva is more highly developed than the adult, so the adult shows degeneration of larval organs. It occurs in ascidians (Urochordata): the free-swimming tadpole larva loses its notochord, tail and sense organs and becomes a sessile, degenerate adult.

j. Kala jar Kala jar (kala-azar, visceral leishmaniasis) is a serious disease caused by the flagellate protozoan Leishmania donovani, transmitted by the bite of the female sandfly (Phlebotomus). The parasite multiplies in macrophages of the spleen, liver and bone marrow, causing prolonged fever, enlarged spleen and liver, anaemia, weight loss and darkening of the skin. It occurs in the Terai region of Nepal and is treated with drugs such as liposomal amphotericin B.

Protozoa

Protozoa are single-celled, eukaryotic, microscopic animals. They have no cell wall, are generally motile (by pseudopodia, flagella or cilia), and reproduce mainly by binary fission. They may be free-living or parasitic. Examples: Amoeba, Paramecium, Trypanosoma, Leishmania, Plasmodium.

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Leishmania donovani

Systematic position

Habitat and morphology

It is an intracellular parasite of the reticulo-endothelial system (liver, spleen, bone marrow, lymph nodes) of man. It occurs in two forms:

1. Amastigote (LD body) — in man. Round/oval, 2–4 µm, no free flagellum; nucleus and kinetoplast (rod-shaped) present. Multiplies by binary fission inside macrophages.
2. Promastigote (leptomonad) — in the sandfly gut and in culture. Slender, 15–25 µm, with a free anterior flagellum arising near the kinetoplast.

Life cycle (digenic, two hosts)

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

1. In man (asexual cycle): The infective promastigote is injected by the bite of an infected sandfly. It enters macrophages and transforms into the amastigote, which multiplies by binary fission until the cell ruptures. Liberated amastigotes infect fresh macrophages. This causes visceral leishmaniasis (kala-azar).
2. In sandfly (sexual/developmental cycle): The fly sucks blood containing amastigotes. In the midgut these become promastigotes, multiply by longitudinal binary fission, migrate to the pharynx and buccal cavity, and are ready to infect a new host in about 6–8 days. The fly remains infective for life.

Thus man is the definitive host (harbours the adult/asexual stage) and the sandfly is the intermediate host and vector.

Pathogenicity — kala-azar (visceral leishmaniasis)

• Incubation period: 2–4 months (range 10 days to 2 years).
• The parasite destroys macrophages of the liver, spleen and bone marrow, producing:
  • Prolonged irregular fever with double rise (twice daily).
  • Splenomegaly and hepatomegaly — spleen becomes enormously enlarged and firm.
  • Progressive anaemia, leucopenia and thrombocytopenia due to bone-marrow involvement.
  • Emaciation, weakness, loss of appetite, darkening of skin — hence the name kala-azar (black fever).
• Complications: secondary bacterial infections (pneumonia, tuberculosis, dysentery) are the usual cause of death; untreated cases are fatal in 1–2 years.
• Post-kala-azar dermal leishmaniasis (PKDL): after recovery, painless nodular skin lesions appear, which act as a reservoir of infection.
• Other forms: L. donovani may also cause simple cutaneous or mucocutaneous lesions in some regions.

Control measures

1. Treatment of patients (case detection and cure):

• Pentavalent antimonials — sodium stibogluconate (SSG), meglumine antimoniate.
• Second line: amphotericin B (liposomal), miltefosine, paromomycin.
• Complete treatment prevents PKDL and breaks transmission.

2. Vector (sandfly) control:

• Residual insecticide spraying (DDT, malathion) on walls of houses and cattle sheds.
• Use of insecticide-treated bed nets, screening of windows and doors.
• Filling cracks and crevices, plastering walls, removing damp organic matter and rubbish near houses.
• Personal protection: repellents, long sleeves, avoiding sleeping on the floor outdoors.

3. Reservoir and animal control:

• Detection and treatment or destruction of infected dogs; control of stray dogs.
• Rodent control in endemic areas.

4. Environmental and social measures:

• Improve housing, sanitation and drainage; keep cattle sheds away from living rooms.
• Health education about the mode of transmission and early reporting of fever with splenomegaly.

5. Surveillance and prophylaxis:

• Active case search in endemic districts, epidemiological surveillance and reporting.
• No effective vaccine is available at present; research on recombinant and killed vaccines is in progress.

6. National programme: In Nepal, kala-azar is a notifiable disease under the National Kala-azar Elimination Programme (target: elimination as a public health problem, incidence < 1 per 10,000 at district level), using active case detection, free diagnosis by rK39 strip test and free treatment.

Metazoa

Metazoa (Eumetazoa together with Parazoa) is the major group of multicellular animals. Its members are:

• multicellular, with cells organised into tissues (the sponges, Parazoa, show only a cellular grade of organisation);
• heterotrophic, with no cell walls and no plastids;
• diploid, with gametes the only haploid stage; development begins from a zygote through cleavage, blastula and gastrula stages;
• usually motile at some stage, with nervous and muscular coordination in most forms.

Metazoa is divided into Parazoa (phylum Porifera) and Eumetazoa (all other phyla).

Reproduction in Scypha (= Sycon)

Scypha is a marine sponge of phylum Porifera, class Calcarea. It reproduces both asexually and sexually.

Asexual reproduction

• Budding: a bud arises from the base, develops a spongocoel and an osculum, and may separate or remain attached to form a colony.
• Regeneration: a fragment or lost part can regenerate the whole sponge.
• Gemmules are formed mainly in freshwater sponges (not typical of Scypha).

Sexual reproduction

• Scypha is monoecious (hermaphrodite) but protogynous, which prevents self-fertilisation.
• Sperms develop from choanocytes (or archaeocytes); ova develop from archaeocytes in the mesohyl.
• Sperms leave through the osculum, enter another sponge with the water current and are carried to the ovum by a choanocyte. Fertilisation is internal, forming a zygote.

Embryogeny of Scypha

1. Cleavage: the zygote divides by unequal holoblastic cleavage, giving small micromeres and large macromeres.
2. Coeloblastula: a hollow blastula forms, with micromeres at one end and macromeres at the other.
3. Amphiblastula larva: the micromeres develop flagella, and the larva escapes from the parent through the osculum and swims about freely.
4. Gastrulation and settling: the larva attaches by the anterior (flagellated) end. The flagellated micromeres invaginate into the macromeres (emboly), giving a gastrula with a blastopore.
5. Metamorphosis: the micromere layer becomes the inner choanocytes and the macromeres become the outer pinacocytes, with archaeocytes, spicules and mesohyl appearing between. The ostia, spongocoel and osculum form, and the young sessile sponge grows into the adult.

Wuchereria bancrofti — Life Cycle, Pathogenicity and Control Measures

Systematic Position

Phylum: Nematoda (Aschelminthes) → Class: Secernentea → Order: Spirurida → Family: Onchocercidae → Genus: Wuchereria → Species: bancrofti

It is a filarial nematode that causes lymphatic filariasis (elephantiasis) in humans. It is a digenetic (dioecious) endoparasite — the adult lives in the human lymphatic system and the larva develops in a mosquito.

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1. Life Cycle

W. bancrofti is heteroxenous, requiring two hosts:

• Definitive host: Man (adult worm and microfilariae)
• Intermediate host / vector: Female mosquito — mainly Culex quinquefasciatus (also Aedes and Anopheles in some regions)

Steps of the life cycle:

1. Entry into man: An infected female mosquito bites a healthy person and injects third-stage infective larvae (L₃) into the skin. The larvae enter the lymphatic vessels and lymph nodes.

2. Development in lymphatics: The larvae moult twice (L₃ → L₄ → L₅) and become adult worms in about 6–12 months. Adults are thread-like, whitish, 4–10 cm long; the female is longer than the male. They live coiled in the lymph glands and vessels, mainly of the lower limbs, groin, scrotum and pelvic region.

3. Production of microfilariae: The fertilized female is viviparous and liberates microfilariae (embryos) into the lymph, which pass into the blood. Microfilariae are 250–300 µm long, sheathed, with a curved tail.

4. Periodicity: Microfilariae show nocturnal periodicity — they appear in the peripheral blood mainly between 10 p.m. and 2 a.m., coinciding with the biting habit of the Culex mosquito. During the day they retreat to the deep capillaries of the lungs.

5. Uptake by mosquito: When the mosquito bites an infected person at night, it sucks up microfilariae along with blood into its stomach.

6. Development in mosquito: In the mosquito's stomach the sheath is shed; the microfilaria penetrates the stomach wall into the thoracic muscles and develops through L₁ → L₂ → L₃ in about 10–14 days (at 27–30 °C).

7. Migration to proboscis: The infective L₃ migrates to the labium/proboscis of the mosquito. When the mosquito bites a new person, the larvae are deposited on the skin and enter through the puncture wound — completing the cycle.

Note: Man is the only reservoir of infection; there is no multiplication in the mosquito — it acts only as a developmental host.

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2. Pathogenicity (Disease Produced)

The disease is lymphatic filariasis, commonly called elephantiasis because of the enormous swelling of the affected parts.

A. Mechanism of pathogenesis The adult worms live in lymph vessels and nodes, causing mechanical blockage and allergic-inflammatory reactions. This leads to lymphangitis and lymphadenitis, obstruction of lymph flow, and finally chronic lymphoedema.

B. Clinical stages

C. Effects of microfilariae Microfilariae themselves are relatively harmless, but their presence causes eosinophilia and tropical pulmonary eosinophilia in some cases. The main damage is due to adult worms and the host's immune response.

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3. Control Measures

A. Control of the vector (mosquito)

1. Anti-larval measures — filling or draining stagnant water, clearing blocked drains, use of larvicides (temephos, kerosene oil) and biological control with Gambusia fish.
2. Anti-adult measures — indoor residual spraying with insecticides (DDT, malathion), use of mosquito nets (especially insecticide-treated bed nets), screening of houses and use of repellents.

B. Control of infection in man

1. Treatment of patients — diethylcarbamazine citrate (DEC) is the drug of choice; ivermectin and albendazole are also used in mass drug administration (MDA) programmes.
2. Mass drug administration — annual single-dose DEC + albendazole to the whole community in endemic areas, as adopted by Nepal's National Filariasis Elimination Programme.
3. Surgical care — hydrocelectomy and lymphoedema management (elevation, compression, hygiene) in chronic cases.

C. Protection of the individual

1. Avoid mosquito bites by using nets, repellents and protective clothing, especially at night.
2. Avoid sleeping in the open near stagnant water in endemic areas.

D. General / public health measures

1. Health education about the mode of transmission and the importance of early treatment.
2. Improvement of sanitation and safe water supply to reduce breeding sites.
3. Surveillance, case detection and periodic blood surveys (night blood smear examination) in endemic regions.
4. Elimination target — WHO's Global Programme to Eliminate Lymphatic Filariasis (GPELF) aims at interruption of transmission through repeated MDA rounds.

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Conclusion

Wuchereria bancrofti completes its life cycle between man and the mosquito, causing lymphatic filariasis with disfiguring elephantiasis in its chronic stage. Since there is no effective vaccine, control depends on breaking the man–mosquito–man cycle by vector control, mass chemotherapy and health education.

Sitophilus oryzae (the rice weevil) is a serious stored-grain pest. Note: the genus is spelled Sitophilus, not Stiophilus.

1. Systematic position

2. Life history

Type: Complete metamorphosis (holometabolous) — egg → larva → pupa → adult.

Egg. The female bores a small hole in the grain with her snout, lays a single egg inside, and seals the hole with a gelatinous plug. One female lays about 300–400 eggs over her life (roughly 4 eggs per day). Egg period: 3–6 days. Eggs are oval, white, and about 0.5 mm long.

Larva. Legless, white, fleshy, C-shaped grub with a brown head. It feeds on the endosperm inside the grain and completes 4 larval instars. Larval period: about 18–25 days. The whole larval life is passed hidden inside a single grain, so infestation is not visible from outside.

Pupa. Pupation occurs inside the hollowed grain in a chamber prepared by the larva. Pupa is white, exarate (appendages free), and lasts 6–8 days.

Adult. The adult is a small reddish-brown to dark brown beetle, 2.5–3.5 mm long, with a long curved snout (rostrum) and four reddish spots on the elytra. It chews the grain from outside. Adults live 4–8 months (up to about a year) and are strong fliers.

Total life cycle: about 30–40 days in warm humid conditions (optimum 27–30 °C and 70–80% relative humidity). Several generations occur in a year (up to 6–8). Breeding stops below about 15 °C. The insect survives the off-season inside stored grain and in cracks and crevices of godowns.

3. Economic importance

A. Harmful effects (pest status)

1. Primary pest of stored grain — attacks rice, wheat, maize, barley, sorghum, and their products; both adults and larvae damage the grain.
2. Quantitative loss — heavy weight loss of stored grain; losses in Nepal commonly reach 5–10%, and much more in poorly stored stocks.
3. Qualitative loss — grain becomes hollow, floury, and unfit for seed and food; nutritive value falls.
4. Loss of germination — the embryo is destroyed, so infested grain cannot be used as seed.
5. Reduced market value — infested grain fetches a lower price and is rejected in trade.
6. Secondary infestation — the holes made by weevils allow entry of secondary pests and storage fungi, causing mustiness and mycotoxin (aflatoxin) contamination.
7. Vector of pathogens — carries spores of storage fungi and bacteria.
8. Cost of control — expenditure on fumigants, insecticides, and storage structures.

B. Useful aspects

1. Serves as food for insectivorous birds, lizards, and predatory insects in the food chain.
2. Used as a laboratory animal in entomology, toxicology, and insecticide-resistance studies.
3. Its larvae and adults are a source of animal protein and are used as feed in some places.
4. Helps in recycling of organic matter in nature.

4. Control measures (brief)

• Dry grain to below 12–14% moisture before storage.
• Clean and fumigate godowns; store in airtight containers (metal bins, silos, hermetic bags).
• Use residual insecticides (malathion dust) and fumigants (aluminium phosphide tablets).
• Mix inert dusts or botanical powders (neem, tobacco) with seed grain.
• Regular inspection and solar heating of grain.

Conclusion. Sitophilus oryzae is a major primary pest of stored cereals whose hidden larval feeding causes both quantitative and qualitative losses; its control depends mainly on proper drying, clean and airtight storage, and timely fumigation.

Alimentary canal of Unio

Unio (freshwater mussel, phylum Mollusca, class Bivalvia) is a filter feeder. Its alimentary canal is a complete tube with the following parts.

1. Mouth: a transverse slit between the anterior adductor muscle and the visceral mass, bordered by two pairs of labial palps. There is no head, radula or buccal cavity.
2. Oesophagus: a short, narrow, ciliated tube leading from the mouth to the stomach.
3. Stomach: a large sac embedded in the visceral mass, lined with ciliated epithelium. It contains the crystalline style (a gelatinous rod rotated by cilia in the style sac) which projects against a chitinous gastric shield. Ducts open into the stomach from the digestive diverticula (digestive gland / "liver"), which are paired, branched and glandular.
4. Intestine: a long, coiled tube beginning at the style sac and ending in the rectum, with a longitudinal fold, the typhlosole, increasing the absorptive surface.
5. Rectum: the terminal part passes through the pericardium and the ventricle of the heart and runs along the posterior adductor muscle.
6. Anus: opens into the cloacal (suprabranchial) chamber, from where faeces leave through the excurrent siphon.

Mode of digestion

• Feeding: the cilia on the gills (ctenidia) create a water current that enters through the incurrent siphon. Food particles (plankton, detritus) are trapped in mucus on the gills and carried to the labial palps, which sort them: small particles go to the mouth and large ones are rejected as pseudofaeces.
• Extracellular digestion: in the stomach, enzymes (amylase, cellulase, lipase, with others from the digestive diverticula) released by the dissolving crystalline style break down starch and other food. The rotating style also winds the mucus string with food and mixes it.
• Intracellular digestion: fine particles are taken into the cells of the digestive diverticula by phagocytosis, and digestion is completed inside the cells.
• Absorption: digested food is absorbed mainly in the digestive diverticula and the intestine, into the blood sinuses.
• Egestion: undigested matter is formed into faecal rods in the rectum and expelled through the anus into the cloacal chamber and out of the body by the excurrent siphon.

Larval forms in Echinodermata

Echinoderms have indirect development. The free-swimming larva is bilaterally symmetrical and ciliated, and it undergoes metamorphosis into the pentamerous radially symmetrical adult. The larvae are named after the class they belong to, and the hypothetical ancestral form is the dipleurula.

Dipleurula (hypothetical ancestral larva)

A bilaterally symmetrical, planktonic larva with a single, continuous, looped ciliated band running round the body, and a complete gut. All echinoderm larvae are thought to be modifications of it, which supports the origin of echinoderms from bilateral ancestors.

Larvae of Asteroidea (starfishes)

• Bipinnaria: a free-swimming larva with two ciliated bands (preoral and postoral) that form lobe-like arms, a complete gut (mouth, oesophagus, stomach, intestine, anus).
• Brachiolaria: develops from the bipinnaria by adding three short brachiolar arms with adhesive suckers at the anterior end, with which the larva attaches to the substrate before metamorphosis.

Larvae of Holothuroidea (sea cucumbers)

• Auricularia: an ear-shaped larva with a single continuous sinuous ciliated band, resembling the bipinnaria.
• Doliolaria: barrel-shaped, the ciliated band breaks up into 3-5 transverse rings.
• Pentacula: the final larval stage, with five tentacles; it settles and becomes a young sea cucumber.

Larva of Ophiuroidea (brittle stars)

• Ophiopluteus: has long, slender arms (typically four pairs) supported by calcareous rods, with ciliated bands along them.

Larva of Echinoidea (sea urchins)

• Echinopluteus: has several pairs of long arms (up to six pairs) supported by calcareous skeletal rods and edged with ciliated bands. It has a complete gut and swims by ciliary action.

Larva of Crinoidea (sea lilies)

• Doliolaria: a barrel-shaped larva with ciliary rings; it attaches by an adhesive pit and becomes the stalked pentacrinoid stage.

Summary

Significance

• The bilateral larva shows that echinoderms evolved from bilaterally symmetrical ancestors.
• Larval resemblance to the tornaria of hemichordates suggests a relationship with Chordata.
• Planktonic larvae allow wide dispersal of these otherwise slow-moving or sessile animals.

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) about its morphological characters. For insects, keys are the standard tool for identification because of their enormous diversity (over one million described species).

Types of Taxonomic Keys

1. Dichotomous key (single-access key) The most common type. At each step the user is given two mutually exclusive, contrasting statements (a couplet); choosing one leads to the next couplet until the name of the insect is reached. It is called dichotomous because each couplet offers only two alternatives.

Example (simplified):

• 1a. Wings present ……………………………………… go to 2
• 1b. Wings absent ……………………………………… go to 5
• 2a. One pair of wings (hind wings reduced to halteres) ……… Diptera
• 2b. Two pairs of wings ……………………………………… go to 3

2. Polytomous key (multi-access key) More than two alternatives are given at a step. It is more flexible but less commonly used in classical insect taxonomy.

3. Other forms

• Bracketed key — couplets are numbered and the alternative leads are given in brackets; useful for long keys.
• Indented key — each couplet is indented so that the subordinate choices are visually grouped under the chosen lead; easier to follow but takes more space.
• Computerised / interactive keys — modern multi-access keys (e.g. DELTA, Lucid) where any character can be entered in any order.

Characters Used in Insect Keys

Keys rely on external morphological characters that are constant and easily observable:

• Wings — number, presence/absence, venation, texture (membranous, scaly, leathery).
• Mouthparts — biting–chewing, piercing–sucking, siphoning, sponging, lapping.
• Antennae — filiform, moniliform, serrate, clavate, geniculate, plumose.
• Legs — number of tarsal segments, claws, modifications (jumping, swimming, grasping).
• Eyes — simple (ocelli) and compound eyes, their number and position.
• Metamorphosis — ametabolous, hemimetabolous, holometabolous.
• Body regions — head, thorax, abdomen; presence of halteres, cerci, ovipositor.
• Chaetotaxy — arrangement of bristles and setae.

Principle of Key Construction

1. Characters chosen must be clear, contrasting and non-overlapping.
2. Characters should be constant for the group and not vary with sex, age or season.
3. The key should be dichotomous, sequential and unambiguous.
4. Each couplet must be mutually exclusive so that only one lead fits the specimen.
5. Both leads of a couplet should be placed together (bracketed) or properly indented.

Uses of Taxonomic Keys

• Rapid and accurate identification of unknown insects.
• Basis for classification, faunistic surveys and pest management.
• Helps in quarantine, medical and agricultural entomology.
• Provides a permanent, repeatable record of diagnostic characters.

Limitations

• Work only on complete, adult specimens; damaged or immature stages often fail.
• A wrong choice at any couplet leads to a wrong identification.
• Keys are usually regional, so they may not work outside their area.

In practice, identification of insects is done by following a dichotomous key step by step, checking each couplet against the specimen under a hand lens or stereomicroscope, until the family, genus or species name is obtained.

Modes of Locomotion in Protozoa

Protozoa move by four main methods, depending on the structure involved:

1. Amoeboid movement (pseudopodial) Seen in Amoeba, Entamoeba. The cytoplasm flows into a temporary projection called a pseudopodium (false foot). It involves sol–gel transformation: the inner fluid plasmasol converts to jelly-like plasmagel at the advancing tip, while the rear gel liquefies. The cell thus crawls forward. Pseudopodia are also used for engulfing food (phagocytosis).

2. Ciliary movement Seen in Paramecium, Balantidium. Numerous short, hair-like cilia cover the body. Each cilium beats in a coordinated wave — a fast effective stroke and a slow recovery stroke — pushing the organism forward and also sweeping food particles into the oral groove. Cilia are supported by 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 the organism through water. The flagellum has a 9 + 2 microtubule arrangement (axoneme) and works like a propeller.

4. Gliding (wriggling) movement Seen in Gregarina, Eimeria. These sporozoans have no cilia or flagella. They move by a slow gliding or twisting motion caused by contraction of the body wall (pellicle) and secretion of mucus, often aided by body flexion.

Conclusion: Amoeboid, ciliary and flagellar movements are the chief modes, while gliding is a special adaptation in parasitic sporozoa.

Modes of reproduction in Obelia (5 marks)

Obelia is a colonial marine hydrozoan (phylum Cnidaria) that reproduces by both asexual and sexual methods — a classic example of alternation of generations (metagenesis) between a polyp colony and a medusa.

1. Asexual reproduction (polyp colony stage)

• Budding: The most common mode. Small outgrowths (buds) arise from the body wall of the hydranth (feeding polyp) or from the coenosarc (living tissue of the stem). Each bud develops a mouth and tentacles, and either remains attached to form a branch of the colony or constricts at its base and separates as a new individual.
• Fission and fragmentation: The colony can also multiply when a branch or piece of the stolon breaks off and grows into a new colony.
• Formation of medusae (blastostyle budding): Specialised reproductive polyps called gonangia (modified blastostyles) bud off medusae asexually. The medusae are set free through the gonopore, so this budding is the asexual step that produces the sexual generation.

2. Sexual reproduction (medusa stage)

• Gonads: The free-swimming medusa is dioecious (separate sexes). Four gonads (interradial in position) develop on the radial canals — testes in the male, ovaries in the female.
• Gametes: Gonads produce spermatozoa (male) and ova (female) by meiosis. Gametes are shed directly into the sea water, where fertilization is external.
• Development: The zygote undergoes holoblastic cleavage to form a hollow blastula, which becomes a ciliated, free-swimming planula larva. After a short pelagic life the planula settles on a suitable substratum, loses its cilia, and develops into a hydrorhiza (root-like stolon). From this arises the erect hydrorhiza and hydrocaulus, which buds to form the new polyp colony.

3. Alternation of generations (metagenesis)

The life cycle thus alternates between:

• the asexual polyp colony (diploid, reproduces by budding), and
• the sexual medusa (reproduces only sexually, producing the planula).

The medusa is the sexual phase and the polyp is the asexual phase; the two together complete the life cycle of Obelia.

Control measures of Echinococcus granulosus

Echinococcus granulosus is the dog tapeworm whose larval stage (hydatid cyst) causes hydatid disease in humans and livestock. Its life cycle involves the definitive host (dog) and intermediate hosts (sheep, cattle, goat, pig, and accidentally humans). Control is therefore directed at breaking the cycle at each link.

1. Control of the definitive host (dogs)

• Regular deworming of dogs with praziquantel (5 mg/kg) or arecoline hydrobromide at about 6-week intervals.
• Elimination of stray dogs and strict licensing/registration of pet dogs.
• Confining dogs and preventing them from roaming freely around slaughterhouses and grazing land.
• Not allowing dogs to be fed on raw offal, especially sheep/cattle liver and lungs.

2. Control of the intermediate host (livestock)

• Proper meat inspection in slaughterhouses and condemnation of infected organs (liver, lungs).
• Safe disposal of infected offal by deep burial or incineration — never feeding it to dogs.
• Improving livestock husbandry and avoiding grazing in areas contaminated with dog faeces.
• Vaccination of sheep with the EG95 recombinant vaccine to reduce cyst burden.

3. Control of human infection

• Personal hygiene: washing hands with soap after handling dogs and before eating.
• Avoiding close contact with infected/stray dogs and not letting them lick the face.
• Health education of the community about the mode of transmission, sanitation and the danger of feeding raw offal to dogs.
• Provision of safe drinking water and proper sewage disposal to prevent contamination of food and water with dog faeces.

4. Environmental and administrative measures

• Meat inspection and abattoir hygiene under veterinary supervision.
• Periodic screening of dogs (arecoline purge test) and of high-risk human groups by ultrasound/imaging.
• Early diagnosis and treatment of human cases (surgery, albendazole/mebendazole) to reduce suffering.
• Coordinated public health, veterinary and municipal efforts with legislation for dog control.

Conclusion: Since the cycle cannot be broken by treating humans alone, the most effective strategy is simultaneous deworming of dogs, safe disposal of infected offal and public health education — i.e. interrupting transmission between dog and livestock.

The thorax of the freshwater prawn (Palaemon / Macrobrachium) is fused with the head as the cephalothorax and bears eight pairs of thoracic appendages (thoracopods): three pairs of maxillipeds and five pairs of pereiopods (walking legs). They lie under the carapace and are mostly modified for feeding and locomotion. The basic plan is biramous (protopodite with exopodite and endopodite), but the exopodite is retained in the maxillipeds and lost in the pereiopods.

(a) Maxillipeds (3 pairs) – accessory mouthparts

• First maxilliped: small and flattened; protopodite has two leaf-like endites, the endopodite is small, the exopodite is a flagellum, and the epipodite is a large plate. It holds food and directs water currents.
• Second maxilliped: the endopodite is five-segmented and leg-like, the exopodite is a flagellum, and an epipodite and gill are present. It handles and pushes food towards the mouth.
• Third maxilliped: the longest and pediform (leg-like); the endopodite has several segments, the exopodite is a slender flagellum, and an epipodite and gill are present. It covers the other mouthparts, holds food and helps in cleaning.

(b) Pereiopods (5 pairs) – walking legs

Each is uniramous (exopodite lost) and has seven segments: coxa, basis, ischium, merus, carpus, propodus and dactylus. Epipodites are absent but gills are attached near the coxae.

• First pereiopods: slender and chelate, used for cleaning the antennae and body and for picking up food.
• Second pereiopods: the largest and strongest, chelate; used for catching prey, defence and fighting. The male's pair is longer and heavier.
• Third, fourth and fifth pereiopods: non-chelate walking legs ending in a pointed dactylus. They are used for walking, crawling and burrowing, and the fifth pair also helps in cleaning.

(c) Other points

• The male gonopores open on the coxae of the fifth pereiopods and the female gonopores on the coxae of the third.
• Thoracic appendages carry the gills (podobranchs, arthrobranchs and pleurobranchs) inside the branchial chamber. The movements of the exopodites of the maxillipeds help create the respiratory water current.
• All are jointed, chitinous and moved by striated muscles.

Functions in brief: feeding and food handling (maxillipeds), defence and capture of prey (chelate legs), locomotion (walking legs), and support of gills for respiration.

Social behaviour in insects (also called eusociality) is the highest level of organisation in the animal kingdom, in which individuals of the same species live together in an organised colony, cooperate in work, and divide labour so that some individuals sacrifice their own reproduction to help others. It is best seen in termites, ants, bees and wasps (the order Hymenoptera and the termites).

Characteristics of social insects

1. Division of labour (castes): The colony has different morphological and functional castes — a reproductive caste (queen and king/drones) and a sterile worker caste (workers and soldiers). Each caste performs a fixed duty.
2. Cooperative care of the young: Eggs and larvae are reared collectively by the workers, not by the parents alone. This is called alloparental care.
3. Overlapping generations: Offspring of one generation remain in the colony and help raise the next generation, so parents, offspring and siblings live together at the same time.
4. Reproductive division of labour: Only one or a few individuals (the queen) lay eggs, while the sterile workers give up their own reproduction and help the queen. This is the key feature of eusociality.
5. Colony as a superorganism: The colony behaves as a single unit — it builds nests, collects food, defends itself and regulates temperature as one body.

Examples of social behaviour

• Honey bee (Apis): A colony has one queen, many sterile female workers and a few drones. Workers collect nectar and pollen, build wax combs, feed larvae, guard the hive and ventilate it by fanning their wings. The queen secretes a pheromone ("queen substance") that suppresses ovary development in workers. When the colony becomes crowded, it swarms and a new queen is raised.
• Ants: Colonies contain a queen, sterile workers and soldiers. Workers forage, tend aphids for honeydew, and carry food to the nest. Some ants keep fungus gardens or raid other colonies.
• Termites: The colony has a king and queen, sterile workers and soldiers with large mandibles. They feed on wood and digest cellulose with the help of symbiotic protozoans in the gut. Soldiers defend the nest by blocking tunnels with their heads.
• Wasps: Polistes and other wasps build paper nests from chewed wood and rear brood cooperatively.

Advantages of social life

• Better defence against enemies and predators.
• Efficient collection and sharing of food.
• Care of young and protection of eggs and larvae.
• Division of labour increases efficiency and saves energy.
• Better nest building and temperature regulation in the colony.
• Greater survival of the colony as a whole.

Disadvantages

• Heavy competition for food among colony members.
• Easy spread of diseases and parasites in crowded nests.
• Sterile workers cannot reproduce, so their genes pass on only through the queen.
• The colony is easily destroyed if the queen dies.

Conclusion: Social behaviour in insects is a product of kin selection — workers help close relatives and thereby pass on their own genes indirectly. It shows the highest degree of cooperation, division of labour and self-sacrifice in the animal kingdom.

Molluscs (phylum Mollusca) are the second-largest animal phylum after arthropods, with about 85,000 described living species worldwide. Nepal is landlocked and has no marine molluscs, but its steep altitudinal gradient (about 60 m to 8,848 m), varied climate from tropical to alpine, and many rivers, lakes, ponds, springs and moist forests support a considerable terrestrial and freshwater molluscan fauna.

Diversity of molluscs in Nepal

1. Taxonomic spread: Nepalese molluscs belong to two classes, Gastropoda (snails and slugs, the most numerous) and Bivalvia (freshwater clams and mussels). Over 200 species of land and freshwater molluscs have been recorded, and the true number is probably higher because many areas are under-surveyed.

2. Terrestrial molluscs: Land snails and slugs occur from the Terai plains up into the high Himalaya. Common families include Ariophantidae, Bradybaenidae, Helicarionidae and Subulinidae. Genera such as Ariophanta, Macrochlamys, Bradybaena and Sitala are widely reported, and slugs are found in the moist, cool forests of the middle hills. The Eastern Himalaya, including Nepal's hill forests, is a recognised area of land snail endemism.

3. Freshwater molluscs: Rivers, lakes, ponds and irrigation canals of the Terai and inner Terai support a rich fauna. Gastropod genera include Bellamya, Pila, Lymnaea, Indoplanorbis, Gyraulus and Melanoides; bivalve genera include Lamellidens, Parreysia and Corbicula. The Koshi, Gandaki and Karnali river systems and lakes such as Phewa, Begnas and Rara harbour distinct assemblages.

4. Altitudinal pattern and endemism: Some species are endemic or near-endemic to the Himalayan region. Diversity is highest in the warm, humid Terai and Siwalik foothills, moderate in the middle hills, and falls sharply above about 3,000 m, where only a few cold-tolerant snails persist.

5. Ecological and economic importance: Molluscs are grazers and detritivores and are prey for fish, birds and mammals. Freshwater mussels filter water and indicate water quality. Lymnaea and Indoplanorbis are intermediate hosts of trematodes such as Fasciola and blood flukes (Schistosoma species), so they matter in medicine and veterinary science. Pila and Bellamya are eaten locally in the Terai.

6. Threats: Habitat loss from deforestation, river damming and pollution, pesticide use and over-collection for food and shells are reducing populations. Many species are poorly documented, so their conservation status is largely unknown.

In summary, Nepal's molluscan fauna is moderately rich, ranging from Terai freshwater snails and clams to Himalayan land snails and slugs, but taxonomic study and conservation work remain inadequate.

Pearls are smooth, rounded, lustrous concretions of calcium carbonate (mainly aragonite, CaCO₃) deposited in concentric layers around a small foreign particle, produced by certain molluscs — chiefly pearl oysters (Pinctada species) and freshwater mussels. They are valued as gems and are also known as "organic gems" because they are secreted by a living organism.

Formation of pearls:

1. Entry of an irritant — A foreign particle such as a grain of sand, a parasite, or a piece of shell mantle tissue lodges between the shell and the mantle of the mollusc.
2. Defence response — To protect its soft body from the irritation, the mantle epithelium (the outer layer of the mantle) begins to secrete a protective coating around the particle.
3. Secretion of nacre — The mantle cells secrete nacre (mother-of-pearl), which consists of thin alternating layers of aragonite (crystalline CaCO₃) and conchiolin (an organic protein). This is deposited in concentric shells around the nucleus.
4. Growth of the pearl — Layer upon layer of nacre is added over months to years. The interference of light with these thin layers gives the pearl its characteristic pearly lustre (orient).
5. Harvesting — When the nacre layers are thick enough, the pearl is removed from the oyster.

Types:

• Natural pearls — form accidentally in wild molluscs; now very rare.
• Cultured pearls — form when humans deliberately insert a nucleus (a shell bead) and/or mantle tissue into the oyster; this is the commercial source of most pearls today (e.g., Akoya, South Sea, freshwater pearls).
• Blister pearls — form attached to the inner shell surface rather than free within the mantle.

Key point: A pearl is essentially the same material as the inner shell lining (nacre); it forms as a biomineralization defence response of the mollusc against an irritant.

Origin of Echinoderms (5 marks)

1. Geological age and fossil record. Echinoderms are an ancient group. Their fossil record begins in the Lower Cambrian (about 540 million years ago), and they were already abundant and highly diversified by the Cambrian period. Because they possess a calcareous endoskeleton of plates (ossicles), they fossilise well, and the phylum has one of the richest fossil records among invertebrates. About 13,000 fossil species are known, against roughly 6,000 living species.

2. Early (Cambrian) echinoderms. The earliest echinoderms were not like modern forms. They were mostly asymmetrical or irregular, lacked the typical five-rayed (pentaradial) symmetry, and had a flattened, plate-covered body. Important Cambrian and Ordovician fossil groups include:

• Helicoplacoidea — elongate, spiral-shaped, unsegmented forms.
• Edrioasteroidea — disc-like, attached forms with five curved ambulacra; considered close to the ancestral stock.
• Cystoidea (cystoids) and Blastoidea (blastoids) — stalked, attached forms with arm-like brachioles; abundant in the Ordovician and Silurian.
• Crinoids (sea lilies) — appeared in the Ordovician and still survive.
• Asteroids (starfish) and Echinoids (sea urchins) — appear later, from the Ordovician onwards.

3. Evolutionary trend. The general trend in echinoderm evolution has been:

• from asymmetry/irregularity in the earliest forms to pentaradial symmetry in later and modern adults;
• from a fixed, attached (stalked) life to free-living, creeping or burrowing life in many groups;
• development of the water-vascular system and tube feet as the chief locomotory and feeding organs;
• reduction or loss of the stalk and arms in advanced forms (e.g. sea cucumbers, heart urchins).

4. Origin from a bilateral ancestor. The larvae of all living echinoderms are bilaterally symmetrical, while the adults are radially symmetrical. This strongly indicates that echinoderms arose from a bilaterally symmetrical, free-swimming ancestor and secondarily acquired radial symmetry as an adaptation to a sedentary, bottom-dwelling life. The ancestral stock is generally placed near the Cambrian "carpoid" or homalozoan fossils, which were flattened, asymmetrical, plate-bearing forms.

5. Relationship with chordates. On the basis of embryological evidence — deuterostome development, enterocoelic coelom formation, and a mesodermal skeleton — echinoderms are placed with the Hemichordata and Chordata in the group Deuterostomia. Hence echinoderms are regarded as the closest invertebrate relatives of the chordates, and both are believed to have descended from a common bilateral, deuterostome ancestor in the Precambrian.

Conclusion. Echinoderms originated in the Lower Cambrian from a bilaterally symmetrical, deuterostome ancestor, passed through an early asymmetrical, attached, plate-bearing stage (cystoids, blastoids, edrioasteroids), and later evolved pentaradial symmetry and a free-living habit, giving rise to the five living classes — Crinoidea, Asteroidea, Ophiuroidea, Echinoidea and Holothuroidea.

Affinities of Balanoglossus and Herdmania

Both Balanoglossus (a hemichordate) and Herdmania (a urochordate) are placed in the phylum Chordata, and each shows a mixture of chordate and non-chordate (invertebrate) features. Their affinities are described below.

Affinities of Balanoglossus

Chordate affinities:

1. Presence of a notochord, which is restricted to the proboscis and collar region.
2. Presence of pharyngeal gill slits in the wall of the pharynx.
3. Presence of a dorsal, hollow, tubular nerve cord in the collar region.
4. A short, blind diverticulum of the gut (the stomochord) is regarded as homologous to the notochord.

Non-chordate (invertebrate) affinities:

1. The body is divided into three regions — proboscis, collar and trunk — as in some worms.
2. The nervous system is largely epidermal and intra-epidermal, as in annelids.
3. Excretion is by a glomerulus, resembling the nephridia of annelids.
4. The coelom is enterocoelic and the larva (tornaria) resembles the bipinnaria larva of echinoderms.
5. Absence of a true vertebral column and a well-developed brain.

Because of this combination, Balanoglossus is regarded as a connecting link between invertebrates (echinoderms and annelids) and chordates.

Affinities of Herdmania

Chordate affinities:

1. Presence of a notochord in the tail of the larva.
2. Presence of pharyngeal gill slits in the adult pharynx.
3. Presence of a dorsal, hollow nerve cord in the larval stage.
4. Presence of a post-anal tail in the larva.

Non-chordate (invertebrate) affinities:

1. Presence of a thick, leathery test or tunic covering the body, as in some invertebrates.
2. Absence of a notochord, nerve cord and tail in the adult (they are lost during metamorphosis).
3. The adult is sessile and sac-like, resembling a sponge or a bivalve in general appearance.
4. The coelom is reduced and the body shows a simple organisation.
5. Presence of a branchial basket for filter feeding, as in some invertebrates.

Because the chordate features are clear in the larva but lost in the adult, Herdmania is regarded as a degenerate chordate and shows affinities with both chordates and invertebrates.

Conclusion: Balanoglossus is a connecting link between echinoderms/annelids and chordates, while Herdmania is a degenerate chordate that retains chordate features only in its larval stage. Both therefore justify their inclusion in the phylum Chordata.

a. Principles of ICZN The International Code of Zoological Nomenclature rests on the principles of binomial nomenclature (each species gets a two-part Latin name — genus + species), priority (the oldest valid name is the correct one), homonymy (the same name cannot be used for two different taxa), and typification (each named taxon must have a designated type specimen).

b. Osmoregulation in protozoa Freshwater protozoa live in a hypotonic medium, so water constantly enters by osmosis. They expel excess water by contractile vacuoles, which fill, fuse with the cell membrane and discharge water outside, thereby maintaining a constant internal osmotic balance.

c. Polyps and medusa These are the two body forms of Cnidaria. A polyp is cylindrical, sessile, with the mouth and tentacles directed upward (e.g., Hydra, coral). A medusa is umbrella-shaped, free-swimming, with the mouth and tentacles directed downward (e.g., Aurelia). Both show radial symmetry and a diploblastic body.

d. Morphological adaptations of parasitic helminths Parasitic helminths show: loss of locomotory and digestive organs, degenerate nervous system, well-developed reproductive system (large numbers of eggs), suckers, hooks or spines for attachment, thick protective cuticle against host enzymes, and anaerobic respiration.

e. Adaptive radiations in Polychaeta Polychaetes have diversified into many habitats and modes of life — errant forms (free-moving predators like Nereis), sedentary tube-dwellers (Sabella, Chaetopterus), burrowers (Arenicola), and pelagic swimmers (Tomopteris). This radiation is reflected in the varied forms of parapodia, chaetae and head structures.

f. Pheromones in insects Pheromones are chemical messengers released by insects to communicate with members of the same species. The best-known is the sex pheromone (e.g., bombykol of the silkworm moth), which attracts mates; others mark trails, warn of danger or regulate colony behaviour in social insects.

g. Mouthparts of cockroach The mouthparts of cockroach are of the biting and chewing type and consist of: labrum (upper lip), a pair of mandibles (jaws), a pair of maxillae (with palps), a labium (lower lip) and a hypopharynx (tongue).

h. Shells in gastropods The gastropod shell is a single, spirally coiled, calcareous structure secreted by the mantle. It may be dextral or sinistral, and in forms like Helix it protects the soft body; in slugs the shell is reduced or absent.

i. Gastrotricha Gastrotricha is a small phylum of microscopic, aquatic, unsegmented worms with a ciliated ventral surface (hence "hairy-bellied"). They have a complete gut, a pseudocoelom, and are mostly hermaphroditic.

j. Origin of protochordates Protochordates (Urochordata and Cephalochordata) are regarded as the ancestral stock of vertebrates. They show the basic chordate features — notochord, dorsal hollow nerve cord, pharyngeal gill slits and post-anal tail — at least in some stage of life.