26 



STRUCTURE AND CLASSIFICATION" OF BIRDS 



the way in which the folds of the gut are packed away in the 

 body cavity. Mitchell describes the actual coiling of the gut 

 itself. In its simplest condition the gut of any animal, as is 

 shown in their embryos, is a straight tube, passing from the 

 stomach to the cloaca, supported by a continuous dorsal mesen- 

 tery, the ventral mesentery being in nearly all vertebrates 



defective so far as the intes- 

 tinal region is concerned. 

 Thi^ simple condition is, 

 however, not retained in any 

 existing bird ; in all the length 

 of the tube is to some extent, 

 generally to a large extent, 

 longer than the body. The 

 alligator (fig. 8) offers the 

 ideally simplest condition of 

 a coiled intestine, where ad- 

 ditional length is achieved 

 without any complications 

 of the gut, merely by its 

 being thrown into a series of 

 folds, of which all are more 

 or less alike. So simple a 

 condition as this does not occur in any known bird. But 

 there is more than one type in which this arrangement is 

 retained with but little modification. It is a significant fact 

 that the most primitive arrangement of the folds of the 

 intestine, judged from the crocodilian standpoint, than which 

 we have none other more nearly approximating to the 

 probable reptilian ancestor of birds, is found in birds which 

 other considerations lead us to assign a low position in the 

 avian series. In the accompanying drawing (fig. 9) of the 

 screamer, for example, we have a gut which is but slightly 

 advanced from that of the crocodile. The greater part of 

 the small intestine shows the same series of undifferentiated 

 folds, only the duodenal loop (not missing as a specialised 

 fold in any bird) being separated off from the general coiling. 

 The large intestine, however, differs from the short and 



Fig. 8. — Alligator 



Alimentary Tbact (aftee Chalmers 

 Mitchell). 



