108The gastrointestinal tract consisted of esophagus, stomach, and intestine that ends at the anus. The histological structure of the gastrointestinal tract of numerous fish species generally consists of mucosa, submucosa, muscularis, and serosa. Some differences of histological structures among fish gastrointestinal tracts are related to feeding habits, food, age, body shape, and weight. The gastrointestinal tract of herbivorous fish showed highly significant increase in length compared with that of carnivorous fish. The esophagus of many teleosts is divided into two parts; anterior and posterior parts based on type and thickness of the epithelium and tunica muscularis. The epithelium of the anterior part is stratified cuboidal in type covered with microvilli and microridges with many mucous and club cells. The mucosal epithelium of the posterior part of the esophagus is consisted of mucus secreting simple columnar epithelium that may play a role in pregastric digestion. However, taste buds are present in the esophagus of some species. The stomach of fish possesses different configurations according to feeding habits, while juveniles and adults of a variety of fishes are stomachless. The stomach of catfish is divided into three regions according to structure: cardiac, fundic, and pyloric regions. The cardiac mucosa is characterized by large number of well-defined long folds, occupied by closely packed simple branched tubular gastric glands that lined by one type of cells with granular cytoplasm. The fundic region is characterized by a great number of simple branched tubular gastric glands composed of oxyntico-peptic cells that contains a tubulovesicular network and great number of large mitochondria. The most prominent feature of the fundic region is the presence of large number of gastric pits of different sizes. The pyloric region is characterized by absence of gastric glands. Cyprinid fishes have one of the simplest types of gastrointestinal tract among vertebrates. Those fish species do not possess a true stomach, and even in intestine, there is no differentiation of the intestinal glands or crypts of Lieberkuhn. The absence of the true stomach in carp is replaced by a simple dilatation at the anterior part of intestine called intestinal bulb. The mucosa of the bulb shows numerous folds, arranged in zigzagging-like patterns. The surface epithelium is consisted of simple columnar, enteroendocrine, and goblet cells. No multicellular glands of gastric or intestinal type are found in their lamina propria. The intestine of fish is divided into three portions according to thickness of the wall, 109length of mucosal folds, and thickness of muscularis: anterior, middle (posterior), and rectum. There is no true large intestine and little morphologic differentiation into regions is present. The surface epithelium of the three regions is consisted of enterocytes covered with microvilli with numerous goblet cells.