INTESTINAL TRACT OF MAMMALS. 465 



Meckel's tract. Meckel's tract is long and suspended at the periphery of a nearly- 

 circular expanse of mesentery. It passes into the hind-gut by a short straight portion 

 at the point where the caeca occur. The caeca are most interesting. There is a widely 

 expanded common portion terminating in a pair of short conical cseca with the points 

 turned proximally, whilst distally the unpaired portion passes into the hind-gut. I do 

 not think that there can be any doubt but that these paired structures represent the 

 primitive paired cseca of Mammals. It is specially interesting to notice that in the 

 Dugong and in Rhytina there is a single caecum (Owen, 17). The hind-gut is very 

 long, almost equal in length to the fore-gut, and is differentiated very slightly into 

 what may be regarded as colon and rectum. The portal system is simply a single long 

 trunk which may be regarded as the posterior mesenteric vessel, giving off branches to 

 Meckel's tract and to the hind-gut. 



Family Halicokid^e. 



I have had no opportunity of examining a Dugong, but a good deal of information 

 regarding the alimentary canal can be obtained from Owen (17), Rapp (22), and Home 

 (9). The general character is similar to that of the Manatee. The tract consists of 

 an enormously long, much-looped, combined duodenum and Meckel's tract, and a still 

 longer and more capacious hind-gut. At the junction of the two there is a fairly 

 large, conical, thick-walled caecum. 



Family Rhytinid^e. 



Naturally I have had no opportunity of examining Rhytina; Steller (25) himself, 

 however, has given a general description of the intestinal canal, from which it appears 

 that it was enormously long, with a distinct hind-gut, the proximal portion of which, 

 as well as the large single caecum, was sacculated. 



The pattern of the intestinal tract in the Sirenia, at least so far as Manatus is a 

 guide, is extremely interesting. In the drawing (fig. 15), for the sake of convenience 

 in arranging the actual specimen, the mesentery is represented as twisted on itself in 

 the region where the hind-gut joins Meckel's tract. In reality, the gut is simply two 

 large loops suspended from the dorsal mesentery, one loop representing the duodenum 

 and Meckel's tract, the other representing the colon and rectum. The paired caeca 

 mark the point where each loop returns to the primitive dorsal line. If the caeca were 

 long, the gut would closely resemble that of the Ostrich, one of the most primitive of 

 birds, and it is difficult to regard the Manatee in any other way than as having retained 

 a primitive Reptilian pattern of the alimentary tract. The descriptions by older authors, 

 which I have cited, make it plain that, except as regards the caecum, the intestinal 

 tract of the Dugong and of Steller's Sea-Cow were essentially like that of Manatus. 

 The single caecum in its allies links this Reptilian condition with the condition most 

 usual in Mammals. Amongst other Mammals, the most nearly allied patterns are 

 those displayed by Ilymx and by the Proboscidea. 



