With the exception of _Amphioxus_ and probably also of the _Cyclostomata_, the gland appears to be present in all vertebrates, varying, however, much in size, shape and relation to the intestinal tube. Usually it appears as an elongated, flattened, more or less distinctly lobulated organ, in close apposition to the duodenum between the layers of the mesoduodenum. In all forms in which the gland is found it is connected with the post-gastric intestine and marks the beginning of the midgut. In structure the gland is usually acinous, resembling the salivary glands. It is well developed in the selachians, forming a triangular body connected with the beginning of the midgut (Fig. 202). In some instances the gland elements do not extend beyond the intestine itself, but remain imbedded in the wall of the midgut, as in _Protopterus_. In certain adult teleosts the pancreas is surrounded by the liver (Fig. 196), in others it does not appear as a compact gland but is distributed in the form of finely scattered lobules throughout the mesentery between the two layers of this membrane. On account of this concealed position of the gland it was formerly believed that the adult teleosts did not possess a pancreas. The pyloric caeca (cf. p. 119) found in these forms were consequently considered to be homologous with the pancreas of the higher vertebrates. [Illustration: FIG. 196.--A portion of alimentary canal of _Pleuronectes maculatus_, the flounder, with pancreas attached to biliary duct and concealed in the substance of the liver, which has been removed. (Columbia University Museum, No. 1491.)] In _Myxinoids_ a peculiar lobulated glandular organ is found imbedded in the peritoneal coat of the intestine near the entrance of the bile-duct, into which its lobules open separately. This organ possibly corresponds to the higher vertebrate pancreas. An organ which may represent a dorsal pancreas is also developed in _Ammocoetes_ (larva of _Petromyzon_), but its exact homology is still doubtful. It is possible that a true pancreas has not yet developed in the cyclostomata. In _Amphioxus_ no trace of a pancreas is found. In all other vertebrates the gland is present. In certain amphibians, as the frog, the single pancreatic duct opens into the common bile duct (Fig. 197). [Illustration: FIG. 197.--Pancreas and biliary ducts of _Rana esculenta_, frog. (Wiedersheim, after Parker; both from Ecker.)] In lacertilians and in some chelonians a lateral offshoot of the pancreas is directed transversely and is adherent to the spleen. Fig. 113 shows the gland in _Chelydra serpentaria_. While the gland usually has a single duct, yet two ducts are found in a number of animals (many mammals, birds, chelonians and crocodiles). At times three ducts are encountered, as in the chicken and pigeon. The arrangement of the pancreatic duct system among mammalia presents the following variations: