A STUDY OF THE SKELETON 187 



body; whereas vertebrates are characterized by the pos- 

 session of a bony skeleton within the body. There are, 

 however, exceptions to this general rule. 



Invertebrate Skeletons. Most of us are familiar with com- 

 mon coral in its branching form or in the spherical form 

 which resembles the human brain (Fig. 80). This coral 

 is really a skel- 

 eton formed by 

 coral animals 

 as a means of 

 protection. The 

 depressions in 

 the surface of 

 the 1 skeleton in- 

 dicate the posi- 

 tion and form 

 of the bodies 

 of the animals. 

 Starfishes, too, 

 construct a cov- 

 ering of bony 

 plates in the 

 skin which, al- 

 though hard FIG. 81. Living Starfish, showing Tube Feet pro- 

 ' jecting from the Lower Surface of the Skeleton. 

 can be moved The Mouth is in the Center of the Star, 

 by the living 



animals (Fig. 81). The outside skeleton of the mollusks 

 (snails and clams) takes the form of a single or double 

 shell, within which the animal can withdraw itself, and 

 oftentimes completely close the shell. 



On the exterior of the body of lobsters, crayfishes, and of 

 most insects is a hard covering that incloses the soft parts. 

 As the animal grows, the shell becomes too small, and then 

 it is split along the dorsal surface. The animal then pulls 

 its body out of the shell and withdraws to a protected place 

 until a new hard skeleton is formed. One might compare 



