(a) Structure of gemmules in Porifera and their importance. (10 marks) (b) Structure and functions of parathyroid gland. (10…

(a) Gemmules in Porifera: structure and importance (10 marks)

Structure. A gemmule is an internal bud of freshwater sponges such as Spongilla and Ephydatia, and of a few marine ones. It is a spherical mass of undifferentiated archaeocytes (thesocytes) packed with reserve food. This is enclosed in a tough coat, made of an inner layer of spongin, a middle pneumatic layer with amphidisc spicules (gemmoscleres) set radially, and an outer spongin layer. A small opening, the micropyle (foramen), lets the cells emerge.

Importance.

• Survival: it withstands drought, freezing and other adverse seasons, when the parent sponge dies and disintegrates.
• Asexual reproduction: with the return of good conditions the archaeocytes emerge through the micropyle and grow into a new sponge.
• Dispersal: gemmules are carried by water, wind and animals.
• Taxonomy: the shape of the amphidiscs is a diagnostic character in classification.
• Experimental value: they are used to study development and cell differentiation.

(b) Parathyroid gland: structure and functions (10 marks)

Structure. There are usually four small, oval glands, in two pairs, embedded in the posterior surface of the two thyroid lobes. They arise from the endoderm of the third (inferior gland) and fourth (superior gland) pharyngeal pouches. Each gland has a thin capsule and a rich blood supply. Histologically it contains chief (principal) cells, which secrete the hormone, and fewer, larger oxyphil cells.

Functions. The chief cells secrete parathyroid hormone (PTH), which raises blood calcium.

• Bone: PTH stimulates osteoclasts to resorb bone and release calcium.
• Kidney: it increases the reabsorption of calcium, and the excretion of phosphate, and activates vitamin D to calcitriol.
• Gut: calcitriol increases the absorption of calcium.

Low blood calcium stimulates secretion, and high calcium suppresses it, by negative feedback. Calcitonin opposes PTH. Hyposecretion causes hypocalcaemic tetany, and hypersecretion causes bone decalcification (osteitis fibrosa cystica) and kidney stones.

(c) Retrogressive metamorphosis in Herdmania (10 marks)

Herdmania is a sessile tunicate whose development is called retrogressive because a more advanced larva changes into a less advanced adult.

Larva. The free-swimming, non-feeding tadpole larva shows the chordate characters: a notochord in the tail, a dorsal hollow nerve cord with a brain vesicle containing an otolith and an eye spot, pharyngeal gill slits and a post-anal tail. It lives for a few hours.

Changes.

• Loss: the tail and its notochord are absorbed, the nerve cord shrinks to a single dorsal ganglion, and the sense organs are lost.
• Gain: the larva attaches to a substratum by adhesive papillae, the pharynx grows with many gill slits, atrial and buccal siphons form, and a tunic is secreted, giving a sessile, filter-feeding adult.

Significance. It reveals the chordate affinities of the tunicates and supports the view that vertebrates arose from a larval, tadpole-like ancestor.

(d) Locomotion in echinoderms (10 marks)

Basic mechanism. Movement is by the water-vascular system: seawater enters through the madreporite and passes through the stone canal, ring canal and radial canals to the lateral canals and the tube feet, each with an ampulla. When the ampulla contracts, water is forced into the tube foot, which extends. Its sucker grips the surface, then longitudinal muscles shorten the foot and pull the body forward. Many feet act in a coordinated way.

Class variants.

• Asteroidea (Asterias): tube feet with suckers on the arms, and a leading arm.
• Ophiuroidea (Ophiura): serpentine movement of the flexible arms, and tube feet without suckers.
• Echinoidea (Echinus): spines and tube feet together, and burrowing by spines in sand dollars.
• Holothuroidea (Holothuria): tube feet on the ventral surface, together with body contractions; some burrow.
• Crinoidea (Antedon): feather stars crawl or swim with the arms, whereas sea lilies are sessile.

(e) Parental care in reptiles (10 marks)

Parental care raises the survival of eggs and young.

• Nest making: sea turtles (Chelonia) dig a pit on the beach and cover the eggs, and female crocodilians (Crocodylus) build a mound of vegetation.
• Guarding: female crocodiles and the king cobra (Ophiophagus hannah), which builds a nest of leaves, guard the nest.
• Brooding: the female python (Python molurus) coils round the eggs and shivers, raising the temperature (thermogenesis).
• Young: the mother crocodile opens the nest in response to the calls of the hatchlings and carries them in her mouth to water.
• Viviparity: the eggs are retained inside the mother, as in garter snakes, sea snakes, many lizards and some skinks (Mabuya), which protects the young, and is an adaptation in cold climates.