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 1n 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' 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,” as 
in the Teleosts Amiurus catus, Trigla gurnardus, and Cyclo- 
pterus 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 hes 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.* 
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 
For references see Howes, Linn. Soc. Journ. Zool. xxiii. 1890, p. 381. 
Howes, op. cit. 
Macallum. Reprinted from Proc. Canadian Instit. N.S. ii. 1884, p. 387. 
Howes, op. cit. 
- © to 
