ALl.MKXTAKV TRACT OK CEKTAIX lilltOS. 87 



Reptiles. Indeed, in an earlier paper*, Dr. Mitchell lias himself 

 sketched in a perfectly correct fashion the aliraentaiy tract from 

 an Alligator, illustrating the facts to which I have just directed 

 attention. Fi'om a gut like this it appears to me to be only just 

 possible to distinguish that of Gasuccrivs t in its general charac- 

 teristics, among which I do not include the well-developed cjsca. 

 The only difference that I can detect is a closer approximation 

 between the ileic and duodenal regions in Casuarkis, already, 

 however, mai'ked, though to a less extent, in the Crocodilidae, 

 which, of course, foreshadows the very close association found in 

 all other bii-ds. This association, caused by the outgrowth of the 

 long middle part of the small intestine from a short region of 

 the primitively straight gut, naturally bi-ings about the com- 

 mencement of the formation of the fixed ileic loop, so conspicuous 

 a character of the alimentary tract of other birds. It is most 

 interesting to notice that among Crocodiles there is, in some 

 species at least, a quite distinct ileic loop, related perhaps to this 

 same association between the ileic and duodenal regions, which 

 is not, however, as has been already remarked, so close among 

 Birds. It is to be noted that here as elsewhere the closest asso- 

 ciation of Birds and Reptiles is shown, thoroughly justifying the 

 views of Cope, Huxley, and others. It may be admitted, therefore, 

 that Casuarius is, at any rate, one of those birds whose intestinal 

 tract, both arrangement and convolutions, hardly differs from 

 that of Reptiles, and is therefore primitive as compared with that 

 of many other birds. Nor, indeed, is there so far any very great 

 difference from the most primitive form of the gut in Mammals, 

 where, as in Casuar^iibs and Crocodilus, the entire intestinal tract 

 is borne upon a continuous mesentery. 



§ The Course of the Evolution of the Gut. 



From the simple conditions which obtain in Gasioarius the 

 more complicated intestinal tract of other birds can be derived : 

 and an almost complete chain of intermediate stages is exhibited, 

 even among the few genera which I have had the opportunity of 

 studying. It is from this point that the characters of the intes- 

 tinal tract in Birds diverge from those of Mammals, the Reptilian 

 conditions being left behind by both groups of Vertebrates. It 

 may be convenient at this stage to point out the essential 

 differences which distinguish the intestinal tract of Birds from 

 that of Mammals. It has already been pointed outi that one 

 difference is to be seen in the fact that among Mammals the 

 permanent loops of the large intestine distinguish that gut from 

 the small intestine, where there are no such permanent loops ; 

 whereas in Birds it is the small intestine only which ex- 

 hibits these permanent loops. These characters, however, though 



* P. Z. S. 1896, p. 137, fig. 1. 



t I have cvamined the species C. rotJixr.hiJdi, C. intensus, and C. wesfcrmaitni. 



X E-J; Gcgenbaur, Vergl. Auat. d. Wirbelth. 



