344 COMPARATIVE ANATOMY 



thus sharply marked off from one another, and the distinction 

 between them is rendered still more marked by the sacculations 

 of the anterior part of the large intestine. Only the posterior 

 portion of the latter, or rectum, which passes into the pelvic cavity, 

 corresponds to the large intestine of lower Vertebrates ; the re- 

 maining and far larger part occurs only in Mammals, and is called 

 the colon. 



The caecum, which is almost always present, undergoes various 

 modifications both as to form and size. Thus in Edendates (Manis, 

 Bradypus), many Carnivora, Odontoceti, Insectivora, and Cheirop- 

 tera, it is very small or even entirely wanting, while in Herbivora 

 it may exceed the whole body in length. An inverse development 

 in size is usually noticeable between it and the rest of the large 

 intestine. In many cases (certain Rodents, Anthropoid Apes, and 

 Man) an arrest of a portion of the caecum takes place in the course 

 of individual development, so that little more than the distal end 

 (processus vermiformis) remains (Fig. 227). In Lepus the enor- 

 mous caecum is provided with a spiral valve, and in Hyrax, besides 

 a large sacculated caecum at the junction of the small and large 

 intestines, there is a pair of large, simple, conical caeca further 

 back. 



Monotremes only amongst Mammals possess a distinct cloaca, 

 though in Marsupials and some Rodents and Insectivores (especi- 

 ally in the female) the anal and urinogenital apertures are sur- 

 rounded by a common sphincter. In other Mammals these 

 apertures become completely separated from one another. 



HISTOLOGY OF THE MUCOUS MEMBRANE OF THE ALIMENTARY 



CANAL. 



The epithelium lining the alimentary canal of Vertebrates with 

 the exception of that of the mouth and cloaca, which is usually 

 stratified consists primitively, that is, phylogenetically. of amoeboid 

 or ciliated cells. In some cases this is also true ontogenetically, and 

 in Amphioxus and Protopterus for instance, the ciliated epithelium 

 persists throughout life and in the Lamprey until metamorphosis. 

 In the adult Petromyzon, as well as in many Fishes and even Amphi- 

 bians, ciliated epithelium occurs constantly only in certain parts of 

 the gut, and in the higner Vertebrates cilia are only seen excep- 

 tionally after the embryonic period, so that, as a rule, only ordinary 

 columnar epithelium is present. A striated margin is observable 

 along the free border of the columnar cells, and may be looked 

 upon as a last indication of the earlier ciliated covering : in some 

 lower Vertebrates (e.g. Elasmobranchs, Proteus, Salamander larva?) 

 the individual cells are even capable of an active amosboid move- 

 ment. In this active participation of the cells in the process of 

 absorption an inheritance from primitive Invertebrates can be 



