(i) Some fish do not have a stomach.

(ii) In most fish, there is usually a distinct pylorus present between the stomach and the intestine, which is a constriction in the alimentary canal. There are some fish in which the pylorus is not very distinct, but the common bile duct often passes into the intestine slightly behind the pylorus, and this may also be regarded as marking the line of demarcation.

(iii) The stomach has been present in most extant fishes for some time, and in the evolutionary history of fishes it has been used first for the storage of large quantities of hard-won and time-unsured food, then for the mechanical processing of the food, and finally for the production of digestive enzymes.

(iv) Some true bony fishes also have no stomach, which may be secondary.

Expanded Information:

Digestive system of fish. p>Digestive System of Fish:

The digestive system of fish consists of two main parts: the digestive tract and the digestive glands. In cartilaginous fishes, the rectum opens into the cloaca, which is formed by a slight expansion of the end of the rectum, and the ureter and reproductive ducts open into this cavity. The cloaca opens to the outside of the body with a single cloacal orifice. In scleractinian fishes, the rectum ends with a separate opening for the anus, which is located before the genital pore.

The digestive system has four functions:

Transportation, mechanical processing, chemical processing, and absorption. The digestive system first developed from the protointestines formed by endodermal cells, and in lower vertebrates (fish, cephalopods) the differentiation of the digestive tube is not particularly pronounced, e.g., the digestive tube of sharks, seven-gill eels, there is no differentiation of the large and small intestines, and the esophagus is not separate.

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