(a) What is hypothalamo-pituitary axis? Draw a well-labelled diagram of pituitary gland and describe the functions of its…

Part (a): Hypothalamo-pituitary axis The hypothalamo-pituitary axis is the anatomical and functional link between the hypothalamus and the pituitary gland, through which the brain regulates endocrine activity. It is established by the portal hypophyseal system, a capillary network that carries hypothalamic releasing and inhibiting hormones to the anterior pituitary, and by axons of hypothalamic neurosecretory cells, especially from the supraoptic and paraventricular nuclei, that terminate in the posterior pituitary. The portal system provides a direct route for hypothalamic control, while the neurohypophysis acts as a storage and release site for hypothalamic hormones. The labelled diagram shows the adenohypophysis (anterior lobe) comprising pars distalis, pars intermedia and pars tuberalis, and the neurohypophysis (posterior lobe) comprising pars nervosa and the infundibulum/pituitary stalk. The pars distalis is labelled with GH, TSH, ACTH, FSH, LH and PRL; the pars nervosa with ADH and oxytocin. The anterior hormones act on target organs: GH stimulates growth of bones and muscles and metabolic activity; TSH acts on the thyroid to secrete thyroxine; ACTH acts on the adrenal cortex to release glucocorticoids; FSH and LH act on the gonads to promote gametogenesis, follicle development and steroid secretion; PRL stimulates mammary gland development and milk production. The posterior hormones are stored and released from neurohypophysis: ADH increases water reabsorption in renal collecting ducts and causes vasoconstriction, conserving water; oxytocin contracts myoepithelial cells around mammary alveoli for milk ejection and also stimulates uterine contraction during parturition. Thus the axis integrates neural and endocrine control.

Part (b): Nephridia in Pheretima A nephridium is a coelomic excretory organ that filters coelomic fluid and removes nitrogenous wastes. In Pheretima, three types are present: septal (pharyngeal), integumentary and enteronephric nephridia. The septal nephridia are situated in the septa; their ciliated, funnel-shaped nephrostome opens into the coelom of the segment anterior to their origin, and their ciliated canal leads to a terminal duct that opens to the exterior through a nephridiopore. The integumentary nephridia lie in the body wall; their ciliated nephrostome opens into the coelom of the same segment, and they discharge through a ventral nephridiopore. The enteronephric nephridia differ functionally: they lack a nephrostome and their terminal duct opens into the gut, so excretory fluid is discharged into the intestine. In the first two types, coelomic fluid is collected by the nephrostome, modified in the canal, and discharged externally; the enteronephric nephridia serve the gut rather than the exterior.

Part (c): Respiration in Pila Pila is an amphibious freshwater snail with a mantle cavity that contains two respiratory structures: a ctenidium (gill) used in water and a lung or pulmonary sac used in air. The ctenidium is a single, feathery organ on the right side of the mantle cavity, bearing filaments and leaflets rich in capillaries. In the aquatic phase, water enters the mantle cavity and, by ciliary action, passes over the ctenidial filaments; deoxygenated blood enters the gill through the afferent branchial vessel, oxygen diffuses into the blood in the ctenidial leaflets, and oxygenated blood leaves through the efferent branchial vessel. In the aerial phase, the mantle cavity is filled with air and the vascularized wall of the lung functions as a respiratory surface. The lung is a vascular fold of the mantle cavity. Air enters and leaves through the right-sided pneumostome, whose rhythmic opening and closing by mantle movements regulates gas exchange. The operculum closes the shell aperture, and during estivation Pila may form a calcified epiphragm; metabolism is then greatly reduced and the animal relies on stored air and anaerobic respiration rather than continuous aerial diffusion. These organs reflect adaptation to an amphibious life and illustrate the evolutionary transition from aquatic gill respiration to terrestrial lung respiration.