(a) Explain asexual and sexual modes of reproduction in Paramecium. (20 marks) (b) Write general features of egg laying mammals…

Reproduction in Paramecium combines vegetative multiplication with a sexual process that restores nuclear function and recombines genes. Asexual mode is transverse binary fission in P. aurelia and P. caudatum. The cell elongates, the micronucleus divides by closed mitosis and the macronucleus divides amitotically; this karyokinesis is followed by transverse cytokinesis, in which the cytoplasm, cilia and oral apparatus divide to give two daughter cells. Because the macronucleus is the vegetative nucleus and the micronucleus the germline nucleus, fission rapidly propagates a clone without genetic recombination. The distinction is important: asexual fission is vegetative and preserves the genotype, while conjugation is sexual because it involves meiosis, nuclear exchange and recombination. Nuclear dimorphism is therefore central: the macronucleus controls somatic metabolism, while the micronucleus supplies the hereditary material for conjugation. Sexual mode is conjugation between two compatible mating types. The cells align and form a cytoplasmic bridge. Each micronucleus undergoes meiosis, producing four haploid pronuclei; three degenerate and the remaining one divides mitotically into a stationary and a migratory pronucleus. The migratory pronucleus of each cell passes to the other, and each fuses with the stationary pronucleus of its partner, so that a diploid synkaryon forms in each exconjugant. The old macronucleus then degenerates; the synkaryon divides to form a new micronucleus and a new macronucleus, after which the exconjugants separate and resume binary fission. Thus conjugation is not merely fertilisation: it replaces the aged macronucleus and introduces recombination. Autogamy and cytogamy are rarer variants of this sexual cycle.

Mammalian groups. Egg-laying mammals (Monotremata/Prototheria) include Ornithorhynchus and Tachyglossus. They lay leathery eggs, have a cloaca, secrete milk onto skin patches without nipples, and the platypus has electroreception. Monotremes thus retain ancestral egg-laying traits while still producing milk. Pouched mammals (Marsupialia/Metatheria) include Macropus and Didelphis. Gestation is short, the young are born altricial, and a chorioallantoic placenta is present but short-lived; the young complete development in a marsupium with teats. Marsupials combine placental pregnancy with pouch nursing. Aquatic mammals include Cetacea, Sirenia and Pinnipedia. Cetaceans have a fusiform body, blubber, forelimbs as flippers, reduced or lost hindlimbs, tail flukes and a dorsal blowhole; Indian examples are Platanista gangetica and Neophocaena phocaenoides. Sirenians are herbivorous, with paddle-like limbs and a tail fluke. Pinnipeds retain hindlimbs modified into flippers, have nostrils on the snout rather than a blowhole, and lack tail flukes; they use blubber and dense fur for insulation. Aquatic mammals show extreme eutherian modification for water. Their aquatic adaptations are therefore group-specific, not a single shared body plan.

Fish scales. Scales are dermal structures arranged in overlapping rows, and their type helps identify the fish group. Cosmoid scales are heavy, layered and fossil, as in coelacanth ancestors. Placoid scales of sharks and rays are dermal denticles with a base, a single backwardly directed tooth-like spine, enamel/enameloid, dentine and a pulp cavity; they protect and reduce drag. Ganoid scales of Polypterus and Lepisosteus are rhomboid, hard and covered by ganoin. Ctenoid and cycloid scales are bony teleost scales: ctenoid have a comb-like posterior margin, cycloid a smooth posterior margin. Diagrammatically, the five types are: placoid as a base with one backward spine, showing enamel, dentine and pulp cavity; ganoid as a rhomboid plate with ganoin; ctenoid as a plate with comb teeth; cycloid as a plate with a smooth edge; cosmoid as a thick layered plate. Placoid scales favour protection and hydrodynamics, while ctenoid/cycloid scales allow flexibility and growth; the evolutionary trend is from heavy ganoid/cosmoid armour to lighter teleost scales. This trend reduces weight while retaining protection. These reproductive and protective adaptations show how organisms balance rapid propagation, genetic renewal, and environmental fit.