(a) Describe the comparative anatomy of digestive system in vertebrates. 20 (b) Give an illustrated account of reproduction in…

(a) Comparative anatomy of the vertebrate digestive system

The vertebrate alimentary canal shows a progressive trend from a simple, straight tube in jawless forms to a regionally specialised tract with elaborate accessory glands in mammals. The comparison is best made class by class.

• Cyclostomes: the mouth is jawless and suctorial; the lamprey has a rasping tongue bearing horny (keratinised) teeth. There is no true stomach, and the intestine is straight with a longitudinal fold (typhlosole). The liver is present, but the adult lamprey loses its gall bladder and bile duct; pancreatic tissue is scattered.
• Cartilaginous fishes: jaws bear rows of replaceable placoid-derived teeth; the stomach is J-shaped; the short intestine carries a spiral valve that greatly increases absorptive surface; the liver is large and oil-rich (aiding buoyancy), and a distinct pancreas and rectal gland are present.
• Bony fishes: the stomach varies with diet and pyloric caeca are common in carnivores; herbivores and detritivores have long coiled intestines. Cyprinids such as Labeo rohita are stomachless, with pharyngeal teeth and a long coiled intestine. The gall bladder is usually present; the pancreas is often diffuse in the mesentery or within the liver (hepatopancreas).
• Amphibians: a wide mouth with small homodont teeth (frogs have maxillary and vomerine teeth) and a protrusible sticky tongue; a short oesophagus, a distinct stomach, a coiled small intestine and a rectum opening into a cloaca. The herbivorous tadpole has a much longer coiled gut than the carnivorous adult. The liver is lobed with a gall bladder, and a compact pancreas lies in the mesentery.
• Reptiles: teeth are homodont (thecodont in crocodilians); the stomach is muscular, and in crocodilians part of it works like a gizzard with swallowed stones; snakes have a long, highly distensible oesophagus and stomach for swallowing whole prey. Herbivorous lizards and tortoises develop a caecum. The intestine opens into a cloaca.
• Birds: teeth are replaced by a horny beak; the oesophagus expands into a crop for storage (secreting "crop milk" in pigeons); the stomach is divided into a glandular proventriculus and a muscular gizzard that grinds food with grit. The intestine is relatively short, with a pair of caeca, and ends in a cloaca. The liver is large and bilobed; the gall bladder is absent in some birds such as pigeons.
• Mammals: dentition is heterodont, thecodont and diphyodont, with salivary glands and a muscular pharynx. The stomach is simple in carnivores and humans but four-chambered in ruminants such as Bos indicus (rumen, reticulum, omasum, abomasum) for microbial fermentation of cellulose; horses and rabbits are hindgut fermenters with a very large caecum. Carnivores have a short intestine, herbivores a long one. A vermiform appendix occurs in humans, some primates and rabbits but is absent in ruminants. The liver is lobed, the gall bladder is present in most mammals (absent in the horse and rat), and the pancreas is a compact gland with exocrine acini and endocrine islets (insulin, glucagon).

Three trends emerge: the gut becomes regionally specialised (crop, gizzard, compound stomach, caecum); absorptive surface is increased either by a spiral valve and typhlosole or by longer coiling with villi; and the liver and pancreas become compact, well-defined glands connected to the gut by ducts and to the hepatic portal system.

(b) Reproduction in sponges

Sponges reproduce both asexually and sexually, and their great regenerative power underlies much of their asexual reproduction.

Asexual reproduction:

• Budding: external buds arise as outgrowths of the body wall and either detach or remain attached to form a colony; internal buds (as in some demosponges) are clusters of archaeocytes that develop within the body.
• Fragmentation and regeneration: broken pieces regenerate complete sponges; even cells dissociated through a sieve can re-aggregate into a functional sponge (H.V. Wilson's classic experiments).
• Gemmules: in freshwater sponges such as Spongilla, Ephydatia and Eunapius, archaeocytes laden with food reserves (thesocytes) aggregate, and surrounding amoebocytes secrete a tough spongin coat reinforced by gemmoscleres (amphidiscs), leaving a pore called the micropyle. Gemmules survive winter or drought; on the return of favourable conditions the archaeocytes emerge through the micropyle and grow into a new sponge.

Sexual reproduction:

• Most sponges are hermaphrodites but avoid self-fertilisation by protandry or protogyny. Gametes arise from archaeocytes or from transformed choanocytes.
• Sperm are shed into the water current and drawn into another sponge through the ostia. A choanocyte captures a sperm, loses its collar and flagellum and becomes a carrier cell that transports the sperm through the mesohyl to the egg; fertilisation is therefore internal.
• Most sponges are viviparous, brooding the embryo within the mesohyl, although some are oviparous and release eggs into the water.
• Calcareous sponges such as Sycon form an amphiblastula: a hollow larva with flagellated micromeres at one pole and larger non-flagellated macromeres at the other. Within the parent it inverts so that the flagella face outward; after release and settlement, the flagellated micromeres invaginate to form the choanoderm, and the macromeres overgrow them to form the pinacoderm.
• Most demosponges form a parenchymula: a solid larva with outer flagellated cells and an inner mass of cells. After settlement the flagellated cells move inside and become choanocytes, while the inner cells give rise to pinacocytes and the mesohyl, an inversion of the usual germ-layer position.

(c) Termites: general features, distribution and social organisation

Termites (formerly the order Isoptera, now placed within Blattodea as the epifamily Termitoidae) are soft-bodied, pale, eusocial insects with chewing mouthparts, moniliform antennae and hemimetabolous development. Winged reproductives (alates) have two pairs of similar, long wings that are shed after the nuptial flight along a basal suture. Unlike ants, they have no waist-like constriction, and their workers are immatures of both sexes.

About 3,000 species are known, found mainly in the tropics and subtropics, with fewer in warm temperate regions. They live in wood, soil, subterranean galleries or epigeal mounds. In India Odontotermes, Microtermes and Coptotermes are important agricultural and structural pests, while Odontotermes obesus builds conspicuous mounds.

Social organisation rests on a caste system:

• Primary reproductives: the king and queen found the colony after the nuptial flight; the queen's abdomen becomes enormously enlarged (physogastry) and in mound-building species she may lay thousands of eggs a day in a royal chamber.
• Supplementary (neotenic) reproductives: they develop if the primary pair dies, so the colony survives.
• Workers: sterile, wingless, usually blind; they forage, build and repair the nest, feed the other castes and tend the brood.
• Soldiers: sterile, with enlarged heads; mandibulate soldiers bite with large jaws, while nasute soldiers (Nasutitermitinae) squirt a sticky defensive secretion from a frontal gland.

Colony life is integrated by pheromones, which inhibit reproductive development in nestmates and lay trails, and by trophallaxis, the exchange of food by mouth or anus; in lower termites proctodeal trophallaxis also passes on cellulose-digesting symbionts after each moult. Lower termites harbour hindgut flagellates such as Trichonympha and Holomastigotoides; higher termites (family Termitidae) lack these flagellates and rely on gut bacteria, and the Macrotermitinae (Odontotermes, Macrotermes) cultivate Termitomyces fungus gardens in their mounds. Mounds are ventilated structures that regulate temperature and humidity. Termites are pests, but they are also major decomposers and soil engineers that improve aeration and nutrient cycling.