The oesophagus is occasionally separated from the stomach by a slight constriction, but more frequently the replacement of the squamous epithelium of the oesophagus by the columnar epithelium of the stomach and the appearance of gastric glands in the wall of the latter cavity afford the only distinction between the two regions. The {254}commencement of the intestine is usually indicated by a pyloric "valve" (Fig. 155, A, B), in the form of a ring-like, inwardly projecting thickening of the circularly-disposed muscle fibres of the terminal extremity of the stomach, and usually also by the entrance of the distinct or united ducts of the liver and pancreas; sometimes, as in certain Elasmobranchs and in the Dipnoi, by a special dilatation or "Bursa Entiana" (Fig. 155, A). The rectum, or terminal portion of the intestine, is distinguished from the rest of the gut by its straight course to the cloacal aperture or the anus, and sometimes by an increase in calibre. In _Box vulgaris_ and a few other Teleosts[226] a caecal diverticulum indicates the commencement of the rectum, while in a few cases the pre-rectal portion of the intestine communicates with the enlarged rectal segment by a much constricted valvular orifice which is suggestive of the ileo-colic valve of the higher Vertebrates,[227] as in the Teleosts _Amiurus catus_,[228] _Trigla gurnardus_, and _Cyclopterus lumpus_. The relation of the regional divisions of the intestine in Fishes to those of other Vertebrates are somewhat difficult to determine. If we may regard the "rectal" gland of Elasmobranchs and the intestinal caecum of certain Teleosts as homologous with each other, and with the caecum coli of the higher Vertebrates, then it would seem that by far the greater part of the intestine of Fishes, including that portion in which a spiral valve may be developed, is homologous with the pre-caecal segment of the gut or small intestine in other Vertebrates, and that the post-caecal section, or large intestine, of the latter is represented in Fishes only by that relatively short portion of the gut which lies posterior to the rectal gland or its homologue in Teleosts, the equivalent of the colon of Mammalia being, as in Amphibia, Reptiles, and Birds, practically undifferentiated.[229] In the Cyclostomata the alimentary canal retains much of its primitive simplicity. It pursues a straight course from mouth to anus, and the usual regions are very obscurely indicated. The same remarks apply also to the Holocephali and a few Teleosts, although in these Fishes the limits of the different regions are somewhat more clearly defined. In the Dipnoi (Fig. 155, A), a contracted sigmoid curve between the somewhat dilated stomach and the spacious intestine is the only departure from the straight course of the preceding groups.