(a) Define conjugation. Explain the mechanism of conjugation in Paramecium. Also give its significance. (20 marks) (b) Describe…

(a) Conjugation in Paramecium: definition, mechanism and significance

Definition: conjugation is the temporary union of two individuals of opposite mating types, during which they exchange micronuclear material, after which they separate and each reorganises its nuclei. Mechanism in Paramecium caudatum (one macronucleus and one micronucleus per cell): Step 1: Cells of opposite mating type, usually after several generations of fission under starvation, stick together by their oral surfaces and form a protoplasmic bridge. Step 2: The macronucleus of each conjugant breaks up and degenerates. Step 3: The micronucleus divides meiotically into four haploid nuclei. Three degenerate and one survives. Step 4: The survivor divides mitotically into a stationary female and a migratory male pronucleus. Step 5: The migratory pronuclei are exchanged across the bridge and each fuses with the stationary pronucleus of the partner to form a diploid synkaryon. The conjugants then separate and are called exconjugants. Step 6: The synkaryon divides three times to give eight nuclei. Four become macronuclei and four micronuclei, and three of the micronuclei degenerate. Step 7: Each exconjugant divides twice, so four daughter individuals result, each with one macronucleus and one micronucleus. Significance: It gives genetic recombination, since the pronuclei come from different individuals, producing new variation on which selection acts. It replaces the old macronucleus and restores the normal nuclear organisation after many fissions, which Maupas called rejuvenation. Woodruff's long cultures showed that Paramecium can also be maintained for years through endomixis, a similar reorganisation in a single cell, so conjugation is useful but not the only means of renewal.

**(b) Life cycle of the leech (Hirudinaria)**

Hirudinaria is a protandrous hermaphrodite, so its testes mature before its ovaries, and cross-fertilisation takes place. Step 1: Two leeches come together in the breeding season, which is the monsoon, and sperm are transferred from the male gonopore of one into the female gonopore of the other. Step 2: The eggs are fertilised internally in the ovisacs. Step 3: The clitellum secretes a mucous ring which hardens into a cocoon. As the leech slips out of it, fertilised eggs and albuminous fluid pass into the cocoon, and its ends are sealed. Step 4: The cocoon is deposited in damp soil near the water's edge. Step 5: Development is direct. The embryo feeds on the albumen and no larva is formed. Step 6: After a few weeks the young leeches break out of the cocoon. They resemble the adults, take blood from hosts, grow, and mature sexually after one to two years. Diagram: a flow diagram of copulating adults, fertilisation, clitellar cocoon in soil, embryo within the cocoon, and the young leech, with labels.

(c) Respiration in the cockroach

Respiratory organs: the cockroach, Periplaneta americana, breathes by a tracheal system. Spiracles: ten pairs, two thoracic and eight abdominal, open to the outside and can be closed by valves. Tracheae: a network of tubes with spiral thickenings (taenidia) which prevent collapse, connected by longitudinal and transverse trunks. Tracheoles: fine fluid-filled branches ending close to the cells of the tissues, where oxygen is delivered directly. Mechanism of respiration: Step 1: Dorso-ventral movements of the abdomen alter the volume of the body and pump air. Step 2: Air enters through the thoracic spiracles and leaves mainly through the abdominal spiracles, since valves open and close in turn. Step 3: Oxygen reaches the tissues through the tracheoles by diffusion, and carbon dioxide diffuses outwards along the same route. Step 4: Spiracles are opened only briefly at rest, which saves water. The blood plays almost no part in carrying oxygen.