96 



ANIMAL LIFE 



we know, on insects and snails and worms. The metamor- 

 phosis of the toad is not so striking as that of the butter- 

 fly, but if the tadpole were inclosed in an unchanging 

 opaque body wall while it was losing its tail and getting its 

 legs, and this wall were to be shed after these changes were 

 made, would not the metamorphosis be nearly as extraordi- 



FIG. 47. Metamorphosis of sea- 

 urchin. Upper figure the adult, 

 lower figure the pluteus larva. 



nary as in the case of 

 the butterfly? But in 

 the metamorphosis of 

 the toad we can see the 

 gradual and continuous 

 character of the change. 



59. Metamorphosis among other animals. Many other 

 animals, besides insects and frogs and toads, undergo meta- 

 morphosis. The just-hatched sea-urchin does not resemble 

 a fully developed sea-urchin at all. It is a minute worm- 

 like creature, provided with cilia or vibratile hairs, by means 

 of which it swims freely about. It changes next into a curi- 

 ous bootjack-shaped body called the pluteus stage (Fig. 47). 

 In the pluteus a skeleton of lime is formed, and the final 

 true sea-urchin body begins to appear inside the pluteus, 



