(a) Draw a well-labelled diagram of adrenal gland and discuss the functions of its hormones. (20 marks) (b) Describe the general…

(a) Adrenal gland: structure and functions of its hormones

Structure: the paired adrenal glands lie above the kidneys, each with a connective tissue capsule and two parts of different origin. The outer cortex is mesodermal and the inner medulla is derived from neural crest (ectodermal) cells. Diagram: a section showing the capsule, the three layers of the cortex (zona glomerulosa, zona fasciculata and zona reticularis), the medulla of chromaffin cells, and the central vein, all labelled. Cortex (under ACTH of the pituitary, except aldosterone): Zona glomerulosa secretes the mineralocorticoid aldosterone, controlled mainly by the renin-angiotensin system and potassium. It raises sodium and water reabsorption and potassium excretion in the distal tubules, which maintains blood volume and pressure. Zona fasciculata secretes the glucocorticoids cortisol and corticosterone. They increase gluconeogenesis, breakdown of protein and fat, suppress inflammation and immunity, and help the body resist stress. Zona reticularis secretes weak androgens such as dehydroepiandrosterone, which contribute to pubic and axillary hair and to libido. Medulla: Chromaffin cells under sympathetic stimulation secrete adrenaline (about 80%) and noradrenaline. They produce the fight-or-flight response: faster heart rate, rise in blood pressure, glycogen breakdown, dilated bronchioles and pupils, and blood diverted from viscera to muscles. Disorders: Hyposecretion of cortical hormones causes Addison's disease, with low blood pressure, weakness and dark skin. Hypersecretion of cortisol causes Cushing's syndrome, with central obesity, high blood sugar and a moon face. Excess aldosterone causes Conn's syndrome, with hypertension and low potassium.

**(b) General features and life cycle of *Ascaris***

General features of Ascaris lumbricoides: Habitat: a parasite in the human small intestine, cylindrical and tapering at both ends. Dimorphism: the female is 20 to 40 cm long with a straight tail, and the male is 15 to 30 cm with a curved tail bearing two copulatory spicules. Body: a thick non-cellular cuticle secreted by the hypodermis resists host enzymes, and there is a triradiate mouth with three lips. The worm is pseudocoelomate, with a complete gut and without a circulatory system. Life cycle (one host, monogenetic): Step 1: Fertilised eggs, thick-shelled and with a mamillated coat, pass out with faeces, up to 200,000 per female per day. Step 2: In moist warm soil the egg develops over two to three weeks, until it contains a second-stage rhabditiform larva, the infective stage. Step 3: Man swallows the egg with contaminated food or water. The larva hatches in the small intestine and penetrates the wall. Step 4: Through the hepatic portal vein it reaches the liver, then the heart and lungs. In the lungs it moults twice and grows. Step 5: It climbs the bronchi and trachea, is swallowed, and reaches the small intestine, where it moults for the last time and matures in about two months. Control: sanitary latrines, handwashing and deworming with albendazole.

(c) Skeletal elements of sponges: structure and types

Skeletal elements of sponges are the supporting structures of the mesohyl that keep the canal system open. They consist of spicules, spongin fibres, or both. Spicules: Structure: crystalline rods or stars secreted by sclerocytes, calcareous (calcium carbonate, by calcoblasts, in Calcarea) or siliceous (hydrated silica formed around an organic axial filament, in Demospongiae and Hexactinellida). Size: large megascleres form the framework, and tiny microscleres lie scattered in the tissue. Types by axes: Monaxons are single-axis rods, such as styles and oxeas, found in Calcarea and Demospongiae. Triaxons have three axes crossing at right angles, giving six rays (hexactines), characteristic of Hexactinellida such as Euplectella. Tetraxons have four rays, such as the calthrops and triaenes of Demospongiae, and the four-rayed spicules of some Calcarea. Polyaxons are multi-rayed stars, called asters, that form microscleres in Demospongiae. Spongin fibres: These are flexible, branching scleroprotein fibres secreted by spongocytes, which form a network in Demospongiae. In bath sponges such as Euspongia they make up the whole skeleton, and the sponge is commercially useful. Freshwater sponges like Spongilla have siliceous spicules with spongin binding them.