Evolution of Caeca, Pancreas, intestine, liver and gall bladder

Caeca:

Macro-organisms attack the food of ruminants before gastric digestion but in the typical non ruminant herbivore, microbial action on cellulose occurs after digestion. Rabbits, horses and rats digest cellulose by maintaining a population of micro-organisms in their usually large caecum, adding to this efficiency, a few non ruminant herbivores such as mice and rabbits, eat some of their own faeces to process the remaining material in them like vitamins.

Pancreas:

Every vertebrate has Pancreas. In lampreys and lung fishes it is embedded in the wall of intestine and is not a visible organ. Both endocrime and exocrime tissues are present but the cell composition varies. Pancreatic fluid containing many enzymes empties into the small intestine viz the pancreatic duct.

Intestines:

The configuration and divisions of small and large intestine vary greatly among vertebrates. Intestines are closely related to animals’ type of food, body size and levels of activity e.g. cyclostomes, chordrichthyes fishes and primitive bony fishes have short nearly straight intestines that extend from the stomach to the anus. In more advanced bony fishes the intestine increases in length and begins to coil. The intestines are moderately long in most amphibians and reptiles. In birds and mammals the intestines are longer and have more surface area than those of other tetrapodes. Birds have two caeca, and mammals have single caecum at the beginning of large intestine. Large intestine is much longer in mammals than in birds and it empties into the cloaca in most vertebrates.

Livers and gall bladder:

In vertebrates with a gall bladder it is closely associated with the liver. The liver manufactures bile pigments. Bile salts emulsify dietary fat to ease the enzyme lipase function. Bile pigments result from phagocytosin of red blood cells in the spleen, liver and red bone marrow. Phagocytosis cleaves the haemoglobin molecule, releasing iron and the remainder of the molecule is converted into pigments that enter the circulation. These pigments are extracted from the circulation in the liver and excreted in the bile as bilirubin (green bile). As bile helps in fat digestion, the gall bladder is large is carnivores and vertebrates in which fat is an important part of the diet. It is much reduced or absent in blood suckers, lamprey and herbivores like Teleosts, many birds and rats.