162 COMPARATIVE ANATOMY AND PHYSIOLOGY. 



In Amphibians and Reptiles the oesophagus is ordi- 

 narily wide, and but few adaptive modifications are 

 to be noticed in it. Oligodon, an egg-eating snake, 

 presents a remarkable arrangement. It will be easily 

 seen that, were this animal to break in its mouth the 

 eggs on which it feeds, the greater part of the contents 

 would almost certainly escape. We find, however, 

 that the teeth in the mouth are quite rudimentary, 

 and that through the upper wall of the gullet there 

 project the elongated inferior spines of several of the 

 vertebrse of this region ; the tips of these spines are 

 coated with a substance of great hardness, and the 

 eggs, after they have reached a position in which 

 their contents can be safely disposed of by the animal, 

 are broken by them. 



The most important modification of the ossophagus 

 is to be found in birds, and especially those that are 

 grain-eating ; there is here developed a considerable 

 enlargement, the so-called "crop," which may not 

 (cassowary), or may (pigeon), be provided with special 

 glands in its walls. The object of the former arrange- 

 ment would appear to be merely that of a kind of 

 reserve-pouch, for it is found in fish-eating forms and 

 in birds of prey. On the other hand, the glandular 

 crop is a necessity for such birds as live on food which 

 is so difficult of digestion as is grain* 



Throughout the whole of the series we meet again 

 and again with examples of a stomach only slightly, 

 if at all, distinguished by its size from the rest of the 

 tract ; and this is not always due to the carnivorous 

 habits of the animal, but rather, as we must suppose, 

 to heredity, and to the fact that some other part of 

 the intestine performs the more important part in the 

 functions of digestion. Ceratodus is a striking 

 example of this, for here the stomach is almost in 

 a straight line with the general axis of the body, 

 while the interior of the small intestine is elaborately 



