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

Comparative anatomy of vertebrate digestion. The vertebrate alimentary canal shows progressive differentiation from a simple tube in agnathans to a highly specialized system in mammals. Cyclostomes have a short gut, an intra-oral tongue and keratinized teeth; many lack a true stomach and a well-developed gall bladder. In gnathostomes the jawed mouth, distinct stomach and accessory glands appear; the alimentary tract becomes more differentiated, and the liver and pancreas become more organized. Chondrichthyes have a spiral valve in the intestine; many teleosts have a J-shaped stomach, but cyprinids such as Labeo rohita are stomachless and have a straight intestine. In bony fishes the gall bladder is common; in Labeo rohita the stomach is absent. Amphibians have a short intestine, a liver and pancreas; the liver is large and the pancreas is small. Reptiles show a more muscular stomach; crocodilians have a complete interventricular septum, while Naja naja has a highly modified gut; the liver is larger and the pancreas is more distinct. Birds possess a crop, gizzard, short intestine and cloaca; the liver is large and the pancreas is small. Mammals have differentiated dentition, a complex stomach in ruminants such as Bos indicus, an elongated intestine and a caecum; a vermiform appendix occurs in some mammals but is absent in ruminants. The liver is lobed, the gall bladder is present in most mammals, and the pancreas is well developed; in ruminants the stomach is divided into rumen, reticulum, omasum and abomasum. Accessory glands are progressively developed: the liver, supplied by the hepatic portal system, secretes bile; the gall bladder stores it; the pancreas has exocrine ducts for enzymes and endocrine islets for insulin and glucagon.

Reproduction in sponges. Sponges reproduce asexually by budding, fragmentation and gemmule formation. External buds arise from the outer surface; internal buds develop within the body. In unfavourable conditions, especially in freshwater sponges such as Ephydatia/Eunapius, thesocytes form gemmules: a mass of archaeocytes enclosed in a resistant spicule coat; after dormancy, a single cell emerges and regenerates a sponge. Sexually, sponges are mostly oviparous, some viviparous. Spermatozoa are released into water and, by water currents, enter other sponges through oscula, where they are carried by choanocytes to ova; fertilization is internal. The zygote develops into an amphiblastula, with flagellated micromeres at one pole and non-flagellated macromeres at the other, or into a parenchymula larva, a free-swimming mass of amoeboid cells. In the amphiblastula, the flagellated micromeres become the inner choanoderm and the non-flagellated macromeres become the outer pinacoderm; in the parenchymula, amoeboid cells differentiate into pinacocytes and choanocytes.

Termites. Termites are soft-bodied, polymorphic, hemimetabolous insects, usually cryptic, living in soil, wood or leaf litter. They are distributed from tropical to temperate regions; in India Coptotermes and Odontotermes occur, causing agricultural and structural damage. Social organization is based on castes. The reproductive caste includes the king and queen, and in some species neotenics that can become supplementary reproductives. Sterile castes include workers, which feed, build and care for brood, and soldiers, which defend the colony; soldiers may be mandibulate or nasute. Caste differentiation is regulated by pheromones, and workers exchange food by trophallaxis; in lower termites this may also transfer symbionts. Digestion is often symbiotic: lower termites harbour hindgut flagellates such as Trichonympha and Holomastigotoides that break down cellulose; higher termites such as Odontotermes lack these flagellates and cultivate fungi. Odontotermes builds mounds with chambers for fungal gardens. Termites are pests but also soil engineers, improving aeration and nutrient cycling.

Across all three, evolution shows increasing specialization: vertebrate digestive systems become more compartmentalized, sponges retain simple cell-level reproduction with regenerative gemmules, and termites achieve complex social and symbiotic adaptations for resource use.