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

Conjugation in Paramecium. In lower invertebrates, asexual division is supplemented by sexual processes that restore nuclear balance. Conjugation is the temporary union of two Paramecia of opposite mating types with reciprocal exchange of nuclear material; unlike autogamy, where pronuclei fuse within one cell, and unlike endomixis, where the macronucleus undergoes reduction without another cell, conjugation involves two individuals and produces genetic mixing. The cells meet at the paroral cone/oral groove, form a cytoplasmic bridge, and the micronucleus enters meiotic prophase, metaphase and anaphase. Three of the four haploid products degenerate; the remaining one divides mitotically to give a stationary and a migratory pronucleus. The migratory pronucleus passes through the bridge and fuses with the partner’s stationary pronucleus, forming a diploid synkaryon. The old macronucleus degenerates. The synkaryon undergoes three mitotic divisions to produce eight nuclei; four become macronuclei and four become micronuclei. Each exconjugant then divides twice transversely, giving four daughter Paramecia, each with one macronucleus and one micronucleus. Conjugation’s significance is genetic recombination and the origin of new hereditary combinations, giving survival value beyond binary fission, which only produces genetically identical clones. It restores the normal nuclear relationship after repeated fissions. It has been associated with rejuvenation, but Woodruff’s long-term cultures showed that Paramecium can be maintained by periodic endomixis without conjugation; hence rejuvenation is not an exclusive consequence of conjugation.

Life cycle of leech. Hirudinaria, the Indian cattle leech, has direct development without a free larval stage. Adults are hermaphrodites but cross-fertilization occurs; internal fertilization follows sperm transfer. In the diagram, two aligned leeches show genital pores, spermatophore deposition and hypodermic impregnation: sperm is placed in the partner’s hypodermis and migrates to the oviducts. Breeding is seasonal, usually in warm, wet months when hosts are available; the pattern is linked to water temperature and host availability. The female secretes a silk cocoon, a protective tube attached to a stone or plant, often in water or on emergent vegetation; eggs with albumen are laid inside. The embryo develops directly into a young leech, which emerges from the cocoon, resembles the adult, feeds on blood, and grows by moulting to maturity. The young leech passes through several juvenile stages, increasing in size, segmental differentiation and feeding capacity. The diagram should also show the cocoon wall, developing embryo, and the emerging juvenile.

Respiration in cockroach. In Periplaneta americana, respiration is through a tracheal system. Ten pairs of spiracles—two thoracic and eight abdominal—open to the outside and are controlled by valves that can be closed by sphincter muscles. They lead to tracheae, which branch into tracheoles; the fluid-filled, non-chitinous tracheoles bring gases close to cells, where cellular respiration occurs. Air sacs, especially in the thorax and abdomen, store air and assist ventilation. In small insects or at rest gas exchange may be passive, but in cockroach active ventilation uses abdominal pumping, thoracic pumping and the valve mechanism of spiracles; air sacs help meet the high oxygen demand of flight. To prevent desiccation, cockroaches often show discontinuous gas exchange: spiracles close, open briefly to fill the tracheal system, then close again, limiting water loss. This differs from aquatic arthropods, which use gills, book gills or tracheal gills and exchange gases across moist surfaces in water; the tracheal system shortens the diffusion path by delivering air directly to tissues.

Thus, conjugation, leech development and cockroach respiration show a trend from simple nuclear exchange and direct development to specialized structures that improve genetic recombination and efficient aerial gas exchange. These examples illustrate how lower invertebrates combine simple asexual reproduction and diffusion with specialized nuclear and respiratory adaptations as body size and activity increase.