334 ZOOLOGY 



beyond these are metatarsals and phalanges. Man is 

 peculiar for walking on the whole series from the tarsus 

 on the largest of the tarsal bones, the os calcis, form- 

 ing the heel. When he "trips it on the light fantastic 

 toe" he reverts to the posture of a remote ancestor. 

 The alimen- 6. The alimentary canal or digestive tract of verte- 

 brates does not differ fundamentally from that of all but 

 the lower invertebrate animals. Even the sea urchins 

 and starfish have such a canal, with the same two open- 

 ings for the entrance of food and the ejection of waste, 

 respectively. The stomach is simply an enlargement 

 of this canal, provided with special glands which secrete 

 the gastric juice. The liver, primitively a pouch or sac 

 arising from the digestive tract, becomes a large and 

 complicated organ. Even the lungs originate in the 

 same manner, and are at first simple sacs. At the an- 

 terior end of the alimentary canal, in the mouth, we find 

 the teeth. It can be seen in the sharks that the teeth 

 are structures of the skin, not differing essentially from 

 the spines which may be found on the outer surface of 

 the animal. The number at first is very great, but as 

 evolution proceeds they are reduced and specialized, 

 and become firmly attached to the jaw bones. Ex- 

 treme types of specialized teeth, like those of the ele- 

 phant and the horse, seem to have little in common with 

 the simple conical structures of many fishes and reptiles. 

 Many invertebrates possess teeth of different kinds,, but 

 these are not homologous with those of vertebrates. 

 The nervous 7. In all vertebrates there is a brain, serving as the 

 system chief controlling center of the nervous system, and there- 

 fore of the whole body. The smaller nerve centers, 

 called ganglia, are relatively very unimportant. The 

 brain is continuous with the spinal cord, and both emit 

 a series of nerves which branch and extend to every part 



