(a) Concept and organization of Coelom (10 marks) (b) Metamorphosis in Insects (10 marks) (c) Thyroid gland and its functions (10…

(a) Concept and Organization of Coelom

The coelom is the true secondary body cavity of eucoelomates, completely lined by mesoderm-derived peritoneum, not the primary blastocoel. It lies between the endodermal gut and the body wall, and is organized into a parietal peritoneum against the body wall and a visceral peritoneum around the organs, with coelomic fluid between them. Animals are acoelomate when mesoderm fills the space, as in Platyhelminthes; pseudocoelomate when a persistent blastocoel is not completely mesoderm-lined, as in Nematoda; and eucoelomate when fully mesoderm-lined, as in Annelida, Mollusca, Arthropoda and Chordata. It forms by schizocoely, splitting of mesoderm in protostomes, or enterocoely, outpocketing of the archenteron in deuterostomes. The coelom suspends organs in mesenteries, allows independent movement, acts as a hydrostatic skeleton, and gives space for growth, respiration and circulation. In segmented animals, septa divide it into compartments; in vertebrates it becomes the peritoneal, pleural and pericardial cavities.

(b) Metamorphosis in Insects

Metamorphosis is an abrupt, hormone-controlled reorganization of the insect body from one life stage to another, producing distinct forms for feeding, dispersal and reproduction. In holometabolous or complete metamorphosis, the sequence is egg, larva, pupa and adult; the larva is worm-like or grub-like, the pupa is a non-feeding reorganization stage, and the adult usually has wings, reproductive organs and adult mouthparts. Examples include Bombyx mori (silkworm), Drosophila, butterflies, beetles and flies. In hemimetabolous or incomplete metamorphosis, the sequence is egg, nymph and adult; the nymph resembles the adult, lacks functional wings and reproductive maturity, and molts gradually, as in cockroach and grasshopper. PTTH from the brain stimulates prothoracic glands to secrete ecdysone, which triggers moulting; juvenile hormone maintains larval/nymphal traits, while its decline permits pupal and adult differentiation. This separation of juvenile and adult niches reduces competition and increases ecological success. It is a key feature of insect diversity.

(c) Thyroid gland and its functions

The thyroid is a bilobed, butterfly-shaped endocrine gland in the neck, connected by the isthmus. Its follicular cells secrete thyroxine (T4) and triiodothyronine (T3), stored in the follicle lumen as colloid; parafollicular C cells secrete calcitonin. T3 and T4 increase basal metabolic rate, oxygen consumption, heat production and tissue growth, and are essential for normal brain development, bone maturation and sexual maturity. Deficiency causes cretinism in children and myxedema in adults; excess causes hyperthyroidism. Calcitonin lowers blood calcium by inhibiting osteoclast activity, complementing parathyroid hormone in calcium homeostasis. Iodine is required for T3/T4 synthesis; its deficiency causes endemic goiter, common in Himalayan and sub-Himalayan regions where soil iodine is low. The gland is regulated by TSH from the pituitary and TRH from the hypothalamus, with negative feedback by thyroid hormones. Thus it links metabolism, development and mineral balance. Its disorders therefore affect energy balance, growth, calcium regulation and reproduction.

(d) Hepatic Schizogony

Hepatic schizogony is the pre-erythrocytic asexual multiplication of Plasmodium in the liver of a vertebrate host, usually humans. After an infected Anopheles bite, sporozoites enter the bloodstream and invade hepatocytes, where they become cryptozoites. The cryptozoite enlarges, replicates its nucleus and cytoplasm, and forms a liver schizont containing many motile cryptomerozoites. In some liver cycles, cryptomerozoites may invade hepatocytes and form metacryptozoites, which undergo further schizogony; this is distinct from hypnozoite dormancy. The released merozoites invade red blood cells, beginning erythrocytic schizogony, which often causes the periodic fevers of human malaria. In P. vivax and P. ovale, some sporozoites may remain dormant in the liver as hypnozoites and reactivate later to cause relapse; hypnozoites are not derived from cryptomerozoites. The pre-erythrocytic phase lasts about 8 days in P. vivax and about 5.5–7 days in P. falciparum. Hepatic schizogony is therefore the amplification stage that precedes blood infection and clinical disease.

(e) Aquatic mammals and their adaptations

Aquatic mammals are mammals that live permanently or largely in water, mainly Cetacea (whales and dolphins), Sirenia (dugongs and manatees) and Pinnipedia (seals, sea lions and walruses). General adaptations include a streamlined fusiform body, reduced hair, thick blubber for insulation, salt balance by specialized kidneys, and diving reflex with bradycardia, oxygen storage in blood and muscle, and countercurrent heat exchange in flippers. Cetaceans and sirenians have blowholes, reduced or absent external hindlimbs, and horizontal caudal flukes for propulsion. Pinnipeds differ: they retain external hindlimbs modified as flippers, have nostrils rather than blowholes, and lack caudal flukes. The Ganges River Dolphin, Platanista gangetica, is an Indian freshwater cetacean with reduced eyes, long rostrum and echolocation, showing adaptation to riverine life. These features show convergent evolution with fish, while retaining mammalian traits such as live birth, lactation and endothermy and air breathing in all aquatic mammal orders.