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

Integument and its derivatives. 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 in elasmobranchs and ctenoid or cycloid scales in teleosts, which reduce drag and protect the body. 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 dermal plates covered by epidermal keratin 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. 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.

Polymorphism and 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, but Obelia’s polyp-medusa alternation is genetically timed. In Obelia, polymorphism is seen both in the dimorphic polyp colony and in metagenesis. A diagram of the colony would show many gastrozooids, feeding polyps with tentacles, and fewer gonozooids, reproductive polyps. Obelia is dimorphic, not trimorphic; dactylozooids are absent. The gonozooid, or blastostyle, is the reproductive zooid; it is enclosed in a protective perisarc capsule called the gonangium, so the gonangium is not the zooid itself. 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 strobilation within the gonangium; the ephyrae detach, mature, and release gametes. Thus Obelia shows life-cycle polymorphism: genetically programmed polyp and medusa forms, plus colonial dimorphism between gastrozooids and gonozooids.

Respiration in Sepia. Sepia has a paired 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, branchial hearts at their bases, accessory branchial hearts, and the siphon. Ventilation is produced by rhythmic contraction and relaxation of the mantle muscles: relaxation draws water in, while contraction expels it through the 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 branchial hearts, assisted by accessory branchial hearts, 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.