(a) Give an illustrated account of integument and its derivatives in vertebrates with suitable diagrams. (20 marks) (b) What is…

(a) Integument and Its Derivatives in Vertebrates

Vertebrate integument is a three-layered covering: an ectodermal epidermis, a mesodermal dermis, and a hypodermis of fat and loose connective tissue. In mammals, the hypodermis stores fat and anchors skin to muscle. A diagram of a vertebrate body wall would show the epidermis as a keratinised or mucous epithelium, the dermis with collagen, blood vessels and nerves, and the hypodermis as insulation. Derivatives include ectodermal hair, feathers, glands, claws and keratin sheaths, and mesodermal dermal bone and fish scales. In fish, the epidermis contains mucous cells and the dermis bears scales: placoid scales (dentine capped by enamel-like vitrodentine, homologous to teeth) in elasmobranchs, ganoid scales in Lepisosteus and Polypterus, cosmoid scales in extinct lobe-finned fishes, and thin cycloid or ctenoid scales in teleosts; they protect the body and reduce drag. Amphibians have thin, glandular skin with mucous glands that keep the surface moist for cutaneous respiration. Reptiles have dry, keratinised scales, scutes and claws; the scales are epidermal keratinised folds, underlain by dermal osteoderms in some groups, and prevent water loss. Birds have ectodermal feathers, with rachis, barbs and barbules, for flight, insulation and display, plus keratinised scales on the legs. Mammals have hair, sebaceous and sweat glands, nails, hooves and horns; hair insulates, sweat glands cool the body, and horns combine a bony core with a keratin sheath, whereas the rhinoceros horn is compacted keratin fibres without a bony core and antlers are shed dermal bone. Other keratinised derivatives include beaks and the baleen plates of whales, and dermal chromatophores allow colour change in fishes, amphibians and reptiles. Functionally, the integument protects, receives touch and temperature, regulates heat and water, and in amphibians assists gas exchange. The dermis contains sensory corpuscles, linking the covering to touch and temperature.

(b) Polymorphism with Reference to the Life Cycle of Obelia

Polymorphism is the occurrence of structurally and functionally distinct forms within one species. It may be genetic, where different morphs are inherited, or environmental, where one genotype produces different forms under different conditions. Environmental polymorphism can be induced by temperature or food; in Obelia the polyp–medusa alternation is part of the normal developmental programme. In Obelia, polymorphism is seen both in the dimorphic polyp colony and in metagenesis. The colony has many gastrozooids (hydranths: feeding polyps with a hypostome and tentacles) and fewer gonozooids (reproductive polyps), so the fixed colony is dimorphic; counting the free-swimming medusa as a third zooid, Obelia is commonly described as trimorphic. Dactylozooids are absent. The gonozooid, or blastostyle, is the reproductive zooid; it is enclosed in a protective perisarc capsule called the gonotheca, and the two together form the gonangium. The polyp stage is sessile and asexual, while the medusa stage is free-swimming and sexual. Fertilised eggs develop into planula larvae, which settle and grow into polyp colonies. Gonozooids produce medusae by lateral budding within the gonangium; the medusae escape, mature, and release gametes. Thus Obelia shows life-cycle polymorphism: genetically programmed polyp and medusa forms, plus colonial dimorphism between gastrozooids and gonozooids. The medusae are dioecious, with four gonads on the radial canals, and because both polyp and medusa are diploid this is metagenesis rather than a true alternation of haploid and diploid generations. Polymorphism thus gives division of labour: feeding, asexual budding and sexual dispersal are carried out by different zooids.

(c) Respiratory Organs and Mechanism of Respiration in Sepia

Sepia has a single ventral mantle cavity containing two ctenidia, or gills. Each ctenidium is lamellated, with primary filaments and secondary lamellae that provide a large surface for gas exchange. Water enters through the incurrent aperture, passes over the gills, and leaves through the siphon. A diagram would show the mantle cavity, the two gills, the branchial hearts at their bases, and the siphon. Ventilation is muscular, since cephalopod gills lack cilia. Contraction of the radial mantle muscles (aided by elastic recoil) expands the mantle cavity and draws water in through the wide inhalant opening between the mantle edge and the head; contraction of the circular muscles, with the mantle edge sealed against the head by cartilaginous locking devices, then forces water out through the funnel (siphon). The siphon directs the outflow, which also provides jet propulsion. The siphon is a muscular tube that can be pointed to direct jets. Deoxygenated blood rich in haemocyanin is pumped by the two branchial hearts (one at the base of each gill) through the gill lamellae. Oxygen diffuses into the blood and carbon dioxide out by counter-current exchange, in which water and blood flow in opposite directions, maintaining a diffusion gradient. Counter-current exchange is efficient because the partial pressure of oxygen remains higher in water than in blood along the lamella. The respiratory system is therefore integrated with circulation and locomotion.