INTESTINAL TKACT OF BIRDS. 



203 



to this, is usually elongated and may be similarly twisted. The supra-duodenal loop, 

 the caeca, and the rectum are as in Anseranas. The vast majority of Ducks and Geese 

 that I have examined, however different their size and habits, faithfully reproduce this 

 type. The diagram given for Cygnus atratm, with the most trifling alterations, might 

 serve for Anas, Anser, ^Ex, Chaulelasmus, Bernicla, Dendrocygna, Fuligula, Nesonetta, 

 Tadorna, and doubtless, so constant is the type, for many others. Mergus albellus, the 

 Smew, presents an interesting variation (fig. 23). The duodenum is unusually wide ; 

 Meckel's tract is thrown into a large number of very short loops at the periphery of an 

 almost circular mesenteric fold, but Meckel's diverticulum lies at the apex of a somewhat 

 longer fold lying in the axis of the system. There is a supra-duodenal loop, but it is not 



Fig. 24. 



Intestinal Tract of Spatula dypeata. Lettering as before. 



drained by a " bridging " vein. The colic caeca are paired, but practically non-existent, 

 and Beddard (2. p. 459) mentions an instance where one of the two was absolutely non- 

 existent. At first sight this intestinal tract appears more archecentric than that of any 

 of the other Anseriform birds except the Palamedeae, but I do not doubt that it is an 

 instance of what I term pseudocentric simplicity. The caeca are obviously degenerate, 

 and in other Mergansers they are longer ; and examination of the first part of Meckel's 

 tract shows that it might have been derived by a fusion of the three first minor loops, 

 these having been more complicated than in Cygnus, and more as they are in Spatula 

 (fig. 24). There is a short supra-caecal kink, but it is not supplied by the rectal vein. 

 Spatula dypeata (fig. 24) shows a form of gut which is simply a further elaboration of 



