IXTESTIXAL TRACT OF MAMMALS. 223 



length of tlie ceecum to the colic loop. There are also strong 

 cavo-duodenal and colico-dnodenal ligaments. 



The remarkable similarity of the gut-patterns of the three 

 families of Perissodactyles contrasts strongly with the fact that 

 there is no resemblance between the Perissodactyle and the non- 

 ruminant and ruminant Artiodactyle patterns. The Swine are 

 omnivorous with a tendency towards vegetable diet ; the Hippo- 

 potamus and all other Artiodactyles are, like the Perissodactyles, 

 vegetarian in diet. In all the hind-gut is capacious in relation 

 to the diet, but the pattern, none the less, follows affinity rather 

 than function. 



Order Rodentia. Dijnis cegyptius (text-fig. 21). 



I have little to add to the account I gave in 1905 (Mitchell, 

 1905, tigs. 26-30) of the intestinal gut-patterns displayed by 

 Rodents. The gut tends to be relatively long, no doubt in 

 association with the chiefly vegetarian diet. The duodenal loop 

 is usually very well marked off from Meckel's tract, the latter 

 always being supported on an oval expanse of mesentery, and 

 varying only to the extent to which it displays minor loops. 

 The caecum is usually capacious, long, and sacculated. Remnants 

 of an originally paired condition are frequent. The caecum, 

 especially when long, tends to be coiled in a spiral, and this 

 coiling may involve not only the distal portion of Meckel's tract, 

 but the proximal portion of the hind-gut. 



Even when the gut is relatively short, traces of the spiral 

 condition are frequent, suggesting that in some Rodents, 

 especially small omnivorous types, the gut has been shortened 

 secondarily from the longer condition normal in the gioup. 



The cfecum is placed rather high up on the recurrent limb of 

 the pendant loop. The remaining portion of the latter varies in 

 a remarkable degree, both in species and in individuals. The 

 most common condition is the presence of two rather narrow 

 colic loops, but these may be reduced to a single loop or there 

 may be three (text-fig. 21, C.L. 1, 2, 3). The most proximal 

 loop (O.L. 1) is the portion that tends to be involved in the 

 spiral twisting of the cfecum, and is what has been termed a 

 paiucascal or post-c?ecal loop. The two more distal loops (C.L. 

 2, 3) may be spirally twisted, either separately or together, but 

 in the more connnon case they are untwisted. I cannot regard 

 this occasional spiral ari'angement as indicating any homology 

 between these loops and the spii'al of Artiodactyles, or as 

 suggesting any special affinity between Rodents and Aitio- 

 dactyles. The colic spiral of the Artiodactyles, especially of the 

 Ruminants, is an extremely definite formation, invariably pi'esent 

 in the adult and appearing at a very early stage in embiyonic 

 life. In Rodents it varies from individual to individual, may 

 involve one or two loops, and is often inconspicuous or absent in 

 small or relatively young individuals. The spiral formation that 



