MAN A VERTEBRATE ii 



cities, and whether they build nests or empires, 

 have two eyes, two ears, nose and mouth, all 

 located in the head, and always occupying the 

 same relative position to each other. Inverte- 

 brates may have their brains in their abdomen, as 

 do the mites ; hear with their legs or antennae, as 

 many insects do ; see with their tunics, like the 

 scallops ; and breathe with their skin, as do the 

 worms. The crayfish hears with its ' feelers,' the 

 cricket and katydid with their fore-legs, the grass- 

 hopper with its abdomen, the clam with its ' foot,' 

 and mysis and other low crustaceans have their 

 auditory organs on their tails. 



Man is, then, like the fishes, frogs, reptiles, birds, 

 and quadrupeds, a vertebrate animal. Excepting 

 in his infancy, when he is a quadruped going on 

 all fours, he uses his posterior limbs only for 

 locomotion, and his anterior for prehension and 

 the like. His spinal axis is erect instead of hori- 

 zontal, and his tail is atrophied. But he possesses 

 all of the unmistakable qualities of the vertebrate 

 type of structure — a two-chambered body cavity, a 

 highly developed and dorsally located nerve trunk, 

 vertebrate vitals, a closed circulatory system, a 

 ventral heart, red blood, a head containing sense 

 organs and brain, and a well-ordered internal 

 skeleton, consisting of a vertebral column with 

 skull and ribs and two pairs of limbs, the limbs 

 consisting each of one long bone, two long bones, 

 two transverse rows of irregular bones, and five 

 branches at the end. 



