(a) Explain habitat, habits, general features and life cycle of Wuchereria. (20 marks) (b) Describe different types of canal…

(a) Habitat, habits, features and life cycle of Wuchereria

Wuchereria bancrofti is a parasitic nematode that causes lymphatic filariasis (elephantiasis).

Habitat. Adults live in the lymphatic vessels and lymph nodes of humans, mainly in the legs and genital region. The microfilariae circulate in the blood. The vectors are mosquitoes, chiefly Culex (C. quinquefasciatus in Indian towns), and also Anopheles and Aedes.

Habits. It is an endoparasite. Adults lie coiled together in the lymph channels for 6–8 years, and the females release sheathed microfilariae. These show nocturnal periodicity, peaking in peripheral blood from about 10 pm to 4 am, which matches the night biting of the vector; by day they stay in deeper vessels such as those of the lungs.

General features.

• The body is thread-like, whitish, cylindrical and unsegmented, with a cuticle, a pseudocoel and a complete gut. The sexes are separate.
• The female is 80–100 mm long and 0.24–0.30 mm wide. The male is smaller (about 40 mm × 0.1 mm), with a ventrally curved tail and two unequal spicules.
• The microfilaria is 244–296 µm × 7.5–10 µm, sheathed, with a tapering tail whose tip is free of nuclei.

Life cycle (humans are the definitive host and mosquitoes the intermediate host).

1. A mosquito sucks microfilariae with blood. They shed the sheath in the stomach, bore through its wall and reach the thoracic muscles.
2. There they become a short, thick first-stage larva (sausage stage), then a second-stage larva and finally the slender, infective third-stage (filariform) larva in about 10–14 days. It moves to the proboscis. There is no multiplication in the mosquito.
3. At the next bite, the third-stage larvae escape onto the skin and enter through the bite wound.
4. They reach the lymphatics, moult twice and mature in about 6–12 months. Mated females then release microfilariae into the blood.

Adult worms obstruct lymph flow, which causes lymphoedema, elephantiasis and hydrocele.

(b) Canal systems in Porifera and their functions

The canal system is the network of channels through which water passes in a sponge. The flagella of choanocytes drive a one-way current that enters by ostia and leaves by the osculum. Three types occur, of increasing complexity.

• Asconoid (Leucosolenia): vase-shaped with a thin wall. Water enters through ostia, which are pores formed by porocytes, into a large central spongocoel lined with choanocytes, and leaves by one osculum.
• Syconoid (Sycon, Grantia): the wall is folded into alternating incurrent and radial canals. Water passes ostia, incurrent canals, prosopyles, radial canals lined with choanocytes, apopyles, the spongocoel (lined by flat cells, not choanocytes) and the osculum.
• Leuconoid (Spongilla, Euspongia): the wall is thick and the spongocoel is lost. Water passes ostia, branching incurrent canals, prosopyles, many small flagellated chambers, apopyles, and excurrent canals to one or more oscula. The chambers open into the excurrent canals directly (eurypylous), through a short aphodus (aphodal) or through both a prosodus and an aphodus (diplodal).

Functions:

• Feeding: the current brings bacteria and plankton, which the choanocytes trap and ingest.
• Respiration: oxygen is supplied and carbon dioxide removed by diffusion.
• Excretion: ammonia and other wastes leave in the outgoing water.
• Reproduction: the current carries sperm in and out and expels larvae.

From asconoid to leuconoid, the choanocyte surface grows much faster than body volume. Hence leuconoid sponges can filter more water, grow large and are the commonest.

(c) Structure of the human eye (labelled diagram)

The labelled diagram should show a vertical section of the eyeball with:

• the sclera, choroid and retina as three coats, and the transparent cornea in front;
• the iris with the pupil, and the ciliary body with suspensory ligaments (zonules) holding the biconvex lens;
• the aqueous humour in front of the lens and the vitreous humour behind it;
• the fovea centralis in the macula lutea, the blind spot (optic disc) and the optic nerve.

Structure: the eyeball is a sphere about 24 mm across, in three coats.

• Fibrous coat: the opaque white sclera maintains shape; the transparent, avascular cornea in front is the chief refracting surface.
• Vascular coat: the pigmented choroid nourishes the retina and absorbs stray light. In front it forms the ciliary body, whose muscle moves the lens and whose processes secrete aqueous humour, and the iris, whose circular and radial muscles control the size of the pupil.
• Retina: an outer pigment layer and an inner neural layer with rods (dim light, rhodopsin) and cones (bright light, colour), then bipolar and ganglion cells whose axons form the optic nerve. The fovea has only cones and gives sharpest vision. The blind spot has no photoreceptors.

The lens is transparent and elastic. For near vision the ciliary muscle contracts, the zonules slacken and the lens bulges (accommodation). Aqueous humour between cornea and lens nourishes both; the vitreous humour is a clear jelly behind the lens that keeps the retina in place.