242 DK. p. CUALMERIS MITCHKLL, OX TIJE 



pair is represented by a single caecum, and in this case it happens 

 that tlie surviving caecum is thin-walled and relatively capacious. 

 If the iigure of the cjecum of that bird (Mitchell, 1913, text- 

 fig. 123) be compared with the normal unpaired caecum of 

 Mammals, it will be seen that the resemblance is very close. 



Position of the Ccecum. — The most common position for the 

 caeca in Birds is distad of the pendant loop on the straight 

 portion of the hind-gut close to the cloaca. This position I 

 associate with the ^progressive shortening of the hind-gut, which 

 is a striking feature in avian anatomy as we turn from less 

 specialized to more specialized types. In the lower types, in 

 which the rectum is. relatively longer, the cteca are placed more 

 proximally on the hind-gut. In the Ostrich (Mitchell, 1896, 

 lig. 4), for instance, where the fore-gut and hind-gut ai-e more 

 nearly equal in length, the caeca occupy a position almost identical 

 with that of the paired caeca in the Manatee or the single caecum 

 of the Elephant. In no case, however, are they proximad of the • 

 distal end of the pendant loop. Among Mammals the most 

 frequent position is about the middle of the recurrent limb of 

 the pendant loop. It is a striking coincidence, however, that in 

 the only Bat with a caecum that I have seen, the position is so 

 close to the distal extremity of the pendant loop, that it may be 

 desciibed as occupying an avian position. In Tarsius (according 

 to Klaatsfih, 1892, pi. xxiii. fig. 8) the caecum is not at the distal 

 end of the pendant loop. In the Carnivores, among which, as 

 among Birds, there is a progressive degeneration of the hind- 

 gut, the caecum, although on the recurrent limb of the pendant 

 loop, is very close to its distal extremity. In the Cetacea the 

 other extreme is present ; the caecum lies almost at the proximal 

 end of the recurrent limb of the pendant loop. The various 

 positions of the csjecum in Birds and in Mammals nearly overlap, 

 but the most frequent position in the one case is distad of the 

 pendant loop, in the other somewhere on the pendant loop, a 

 state of affairs congruous with the idea that the various conditions 

 have come about by divergent modification from a common 

 type. 



Form and FanGtion of the Ccectmi. — I have nothing to add to 

 my former discussion (Mitchell, 1905, p. 522). Only in a most 

 general sense can there be said to be a correlation between diet 

 and the presence, length, and capacity of the caecum. There are 

 many exceptions to any general statement, and it seems as if 

 ancestral history were at least as potent a factor as actual diet. 



Secondary Rdations hetireen Pro.vimal andj Distal Portions of 

 the Intestinal Tract. — Two different kinds of connection may 

 exist between proximal and distal regions of the intestinal tract. 

 The connection to which I have [)aid most attention, and of 



