HISTOLOGY OF THE MUCOUS MEMBRANE. 237 



though in Marsupials and some Rodents, the anal and urinogenital 

 apertures are surrounded by a common sphincter. In all the 

 others a complete separation between these apertures takes place. 



HISTOLOGY OF THE MUCOUS MEMBRANE OF THE ALIMENTARY 



CANAL. 



With the exception of the epithelium of the mouth and anus, 

 which is usually stratified, that of the alimentary canal of Ver- 

 tebrates consists primitively, that is, phy log eneti call y, of 

 ciliated cylindrical cells. In some cases this is also true 

 ontogenetically, and in Amphioxus and Petromyzon (Ammo- 

 ccetes), 1 the ciliated epithelium persists throughout life. 



In all other Vertebrates, especially in the higher forms, cilia 

 are only seen exceptionally after the embryonic period, so that, 

 as a rule, only ordinary cylindrical epithelium is present. 



A striated margin is observable along the free border of the 

 epithelial cells turned towards the lumen of the canal ; this is pro- 

 bably to be looked upon as the expression of the earlier ciliated 



NN 



FIG. 193. DIAGRAM OF THE STRUCTURE OF A C<ELENTERATE. 



AK, ectoderm ; IK, endodenn ; Nff, cells of the endodenn which have thrown out 

 amoeboid processes and ingested food-particles ; UD, primary alimentary cavity 

 (archenteron), containing food-particles (N) ; Um, the primary mouth (blasto- 

 pore). 



covering, and in the lower Vertebrates it is capable of an active 

 amoeboid movement (Fig. 194, B). In this active participation of 

 the cells in the process of absorption, we recognise an old inherit- 

 ance from Invertebrates : in this connection the reader is referred 

 to Fig. 193, which represents diagrammatically the structure of 

 a Ccelenterate, in which the endoderm-cells lining the primitive 

 alimentary cavity, or archenteron (UD}, are directly concerned 

 with the taking in of nutritive particles by means of pseudopodia 

 [intracellular digestion]. These endoderm-cells may be com- 



1 It persists throughout the gut in Ammoccetes, and only in certain parts in 

 Petromvzon. 



