55 



Teeth first appear in 'sea-urchins, forming what is 

 called Aristotle's lantern. The univalve shells carry 

 their teeth on their tongue. 



Many of the fish have teeth all over their mouths. 

 The rays have the roof of the mouth paved with a 

 sort of cobblestone-like teeth for crushing shells. 

 Many forms have no teeth, as the sturgeon among 

 fishes. £Tone of the amphibia except the frogs have 

 teeth. The ant-eater and related forms among the 

 mammals have no teeth. The elephant tusk shown 

 is an upper incisor; he has no lower. The tusk of 

 the walrus is a canine. The teeth of the horse, both 

 ancient and modern, of the mylodon, the saw-fish, 

 mastodon, mammoth, and many others will be noted. 



Adaptations for Locomotion. — First come the fixed 

 forms, such as the barnacle and oyster, practically all 

 of which swim freely when young. Then come the 

 swimming forms, from the minute forms that swim 

 by the rapid motion of cilia to the swimming verte- 

 brates, the fishes with the different types of tail, the 

 turtle with his broad fins, the webbed footed birds, 

 mammals like the otter, and the more fish-like mam- 

 mals, such as the porpoise, etc. Then come the digging 

 forms that live in the ground, like the mole, with its 

 broad forepaws. Then come the creeping forms, the 

 gasteropod, snake, caterpillar, thousand-legged worm, 

 earthworm, etc. The .running forms include animals 



