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

(a) Hypothalamo-pituitary axis, pituitary gland and its hormones

The hypothalamo-pituitary axis is the structural and functional link through which the hypothalamus controls the pituitary and so most of the endocrine system. Two routes exist:

• Hypothalamo-hypophyseal portal system. Neurosecretory cells of the hypothalamus release releasing and inhibiting hormones (TRH, CRH, GnRH, GHRH, somatostatin, dopamine as prolactin-inhibiting factor) into capillaries in the median eminence. Portal veins carry them to the anterior pituitary, where they regulate each hormone.
• Hypothalamo-hypophyseal tract. Axons of the supraoptic and paraventricular nuclei run down the infundibulum to the posterior pituitary, where ADH and oxytocin are stored and released.

Hormones and functions:

• GH: growth of bones and muscles and protein synthesis; it raises blood glucose. Too much causes gigantism or acromegaly and too little causes dwarfism.
• TSH: stimulates the thyroid to secrete thyroxine and triiodothyronine.
• ACTH: stimulates the adrenal cortex to secrete glucocorticoids.
• FSH and LH: control gametogenesis and sex steroids; FSH promotes follicle growth and spermatogenesis, and LH causes ovulation and formation of the corpus luteum and stimulates testosterone secretion.
• Prolactin: mammary growth and milk production.
• MSH (pars intermedia): skin pigmentation in lower vertebrates.
• ADH (vasopressin): water reabsorption in the collecting ducts, and a rise in blood pressure; deficiency causes diabetes insipidus.
• Oxytocin: contraction of the uterus at childbirth and milk ejection.

(b) Nephridia of Pheretima

A nephridium is a coiled tubular excretory organ of annelids that removes nitrogenous waste and excess water, in Pheretima from the coelomic fluid and the blood. Pheretima is micronephridial, with hundreds of nephridia in each segment, of three kinds:

• Septal nephridia lie on both sides of the septa from segment 15/16 onward. Each has a ciliated funnel (nephrostome) opening into the coelom, a coiled tube and a terminal duct that leads to a septal excretory canal, then to a supra-intestinal duct, and finally into the intestine through a pore. They are enteronephric.
• Integumentary nephridia lie on the inner side of the body wall from segment 3 backward, in large numbers in every segment. They have no nephrostome and open directly to the outside by a nephridiopore, so they are exonephric.
• Pharyngeal nephridia form paired tufts in segments 4, 5 and 6, have no nephrostome and open into the buccal cavity and pharynx, so they are enteronephric.

The septal and pharyngeal nephridia pass wastes into the gut, where water is reabsorbed, which conserves water in the terrestrial earthworm. The integumentary nephridia excrete directly to the outside and help in osmoregulation.

(c) Respiration in Pila

Pila is an amphibious freshwater snail whose mantle cavity holds two respiratory organs, a gill (ctenidium) for water and a lung (pulmonary sac) for air. The ctenidium is a single comb-like (monopectinate) gill on the right side of the cavity with many vascular lamellae, and the lung is a thin-walled vascular sac on the left side opening by a pneumostome.

In water, a current driven by cilia enters the mantle cavity through the left nuchal lobe, which acts as an inhalant siphon, passes over the gill, and leaves by the right nuchal lobe. Oxygen diffuses into the blood of the lamellae and carbon dioxide into the water. The blood enters through the afferent branchial vessel and oxygenated blood leaves in the efferent vessel to the heart.

In air, the snail rises to the surface and extends the left nuchal lobe as a tube to the surface, drawing air into the pulmonary sac, where gas exchange takes place across its vascular wall; rhythmic mantle movements renew the air. On land or during drought and aestivation, the operculum closes the shell and the snail uses the air stored in the lung. This dual system suits its amphibious life.