254 FISHES CHAP. 



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 x 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, 2 as 

 in the Teleosts Amiurus calus, s Trigla gurnardus, and Cydo- 

 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 Elasmo- 

 branchs and the intestinal caecum of certain Teleosts as homo- 

 logous 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. 4 



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 



1 For references see Howes, Linn. Soc. Journ. Zool. xxiii. 1890, p. 381. 

 - Howes, op. cit. 



3 Macallum. Reprinted from Proc. Canadian Instit. N,S. ii. 1884, p. 387. 



4 Howes, op. cit. 



