Intestinal Mucosa

• The intestinal mucosa produces secretin and pancreozymin, which are controlled by nervous system and regulate pancreatic secretion. Secretin affects flow of enzyme carrying liquids from the pancreas, whereas pancreozymin accelerates flow of zymogens.

Islets of Langerhans:

• In some fishes like Labeo, Cirrhinus, and Channa small islets are present which are separate from pancreas and are found near gall bladder, spleen, pyloric caeca or intestine.
• Such islets are often referred to as principal islets.
• In fish the islets are big and prominent and consist of three kinds of cells.

(i) The beta cells which secrete insulin and take aldehyde fuschin stain,

(ii) Another type of cells are alpha cells, which do not take aldehyde fuschin stain and have two types, A1 and A2 cells, which produce glucagon. The function of the third type of cells is not known. Insulin is secreted by beta cells and regulates the blood sugar level in fishes.