142 THE LIVING WORLD. 



Development of Hard Parts. 



The next step in the history was the development 

 of hard parts for support and protection. This for 

 the first time made the existence of large animals a 

 possibility. Almost immediately after the diver- 

 gence of types of animals, the development of some 

 form of skeleton began. The echinoderms developed 

 plates of lime in their skin ; the mollusks secreted 

 upon their bodies a thick shell of the same material ; 

 some of the hydroids developed an external shell, 

 and the corals a calcareous framework ; the Articu- 

 lata (except the so-called worms) developed upon the 

 outside of their body a light but tough shell of 

 chitin. Such skeletons as these, while they were 

 certainly of great value, were in one respect a disad- 

 vantage. In the mollusks, for instance, they were 

 large and clumsy compared with the size of the 

 animal ; they prevented rapid motion and effectually 

 checked any great increase in size. It was only in 

 later times, when the mollusks got rid of their clumsy 

 shell, that any very great size could be attained, as 

 in the case of the great giant squids. In the Articu- 

 lata the shell, made of chitin instead of lime, was 

 lighter and tougher, and therefore better adapted to 

 activity. As a result, the Articulata succeeded in 

 producing such active and well-protected groups as 

 the insects. But the external shell of insects and 

 crustaceans is not fitted for any large animal, the old 

 trilobites (some of which were over two feet in 

 length) reaching the extreme in this direction. 



A little later another division of animals hit upon 

 a new device for the support and protection of its 



