FEEDING 281 



Stops at the jaw. Possibly, the Narwhal uses its tusk for stirring up fish and 

 other prey at the bottom of the sea, but more probably it is a secondary 

 sexual characteristic comparable in biological significance with a deer's 

 antlers or a man's beard. 



The Gangetic Dolphin, too, which, as we saw in Chapter 9, feeds 

 mainly at the bottom of turbid rivers, probably used its long jaws with their 

 strong teeth for stirring up the mud, and it seems likely that the Eurhino- 

 delphids, well-preserved fossils of which were discovered in Miocene 

 deposits in America, Japan and Belgium, made a similar use of their long 

 beaks. These 14^ to 16 foot long, completely extinct dolphins had a par- 

 ticularly long, almost needle-shaped, upper jaw and a very much shorter 

 lower jaw (Fig. 161). Both jaws carried a long row of pointed teeth from 

 which we may infer that they were mainly fish-eaters. On the other hand, 

 the section of the upper jaw protruding in front of the lower jaw is toothless, 

 and its function is probably to stir up the slimy or sandy bottom. The fact 

 that these animals, like the Narwhal and the River Dolphins, have a fairly 

 long neck with free cervical vertebrae may also be associated with the 

 same phenomenon. 



When we chew our own food, we begin a rather long and complicated 

 chain of processes which together make up the process of digestion. This 

 whole chain of processes takes place inside the alimentary canal, a twisting 

 and turning tube which runs all the way from the mouth to the anus. By 

 the actions of saliva and of gastric and intestinal secretions our food is 

 broken down until it can pass through the wall of the intestine, where it is 

 absorbed by the blood and then assimilated by the body. The secret of 

 this process lies in the effects of certain substances called enzymes (which 

 are contained in the various secretions) on the proteins, fats and carbo- 

 hydrates which constitute our food. 



In Cetaceans, too, the process begins in the mouth. We have already 

 discussed their teeth, baleen and jaws at length, and in connexion with 

 digestion we need only mention one more fact, viz. that, since Cetaceans 



Figure 161. Skull of Eurhinodelphis cocheteuxi du Bus, from the Upper Miocene 

 {Antwerp). Reconstruction by Abel {igog) from material in the Brussels Natural History 

 Museum. The animal lived some 10 million years ago, probably fed on fish, obtained by 

 churning up the bottom with its long, toothless, beak. 



