Fig. 212.—Alimentary canal of Accipenser sturio, sturgeon. Numerous pyloric cæca are bound together to form a gland-like organ. (Columbia University Museum, Nos. 1826, 1827, and 1828.)
The lower left-hand figure shows the mid- and end-gut in section, the latter provided with a spiral mucous valve.
Fig. 208 shows the stomach and the beginning of the midgut with four pyloric cæca in Pleuronectes maculatus, and Fig. 209 the same parts of this animal in section.
Fig. 210 shows the stomach and midgut of Paralichthys dentatus, the summer flounder, with three well-developed conical pyloric cæca. On the other hand in some forms the number of pyloric appendices is enormously increased, while their caliber diminishes. Thus 191 cæcal appendages are found surrounding the beginning of the midgut in Scomber scomber. A well-marked example of prolific development of the pyloric appendages is furnished by the common cod, Gadus callarias (Fig. 211). The appendices are in the natural condition bound together by connective tissue and blood vessels, so as to form a compact organ, resembling a gland (Fig. 211, A), and a similar arrangement is found in Thynnus vulgaris and alalonga, Pelamys and Accipenser (Fig. 212).
In the smaller upper figure on the left the stomach, mid-gut, and pyloric cæca are seen in section, showing the lumen of the latter and their openings into the mid-gut.
In some Teleosts (Siluroidea, Labroidea, Cyprinodontia, Plectognathi and Leptobranchiates) the appendices are entirely wanting. If there are not more than 8-10 appendices they usually surround the gut and empty into the same in a circle. In other cases they are arranged in a single line, or in a double row, opposite to each other ([Fig. 213]). Each appendix may open into the intestine independently, this especially where the number is limited and the individual pouches large (cf. [Figs. 206]-[210]), or several may unite to form a common duct.
Fig. 211, B, shows the appendices in Gadus callarias, the cod, freed by dissection from the investing connective and vascular tissue. It will be noticed that a considerable number of the tubes unite to form ducts of larger caliber which open into the intestine, as seen in the section shown in [Fig. 214].
The pyloric appendices apparently have the same significance as the spiral intestinal fold of the Selachians, Cyclostomes and Dipnœans, i. e., the production of an increase in the area of the digestive and absorbing surfaces of the intestinal mucous membrane. Hence, as stated, the appendices and the spiral fold are found to vary in inverse ratio to each other. Thus, for example, Polypterus ([Fig. 205]) still has a fairly well developed spiral fold and only a single pyloric appendix, while Lepidosteus, with but slightly developed spiral fold, has numerous appendices. It was formerly held that the pyloric cæca and the pancreas were mutually incompatible structures, and that where one is found the other will be wanting.
Hence the appendices were regarded as homologous with the pancreas of the higher forms. Recent observations have shown that this view is not strictly and entirely correct, while at the same time it merits consideration in several respects.