IKKITABILITY 103 



ly is cylindrical and the muscles run lengthwise. 



'hen they contract they pull the animal back against 



le rock to which it is attached. 

 The number, shape and arrangement of muscles vary 



-eatly in different animals. In simple animals they 



*e few in number, they are arranged very simply, and 

 Few movements are possible. In higher animals they 

 ire numerous and they vary greatly in size, shape and 

 irrangement. An infinite variety of movements may 



lerefore be produced by their contraction. 

 The Function of a Skeleton. — Very simple animals 

 are more or less jelly-like and formless. In larger and 

 more complex animals the soft tissue is supported in 

 order that the shape of the body may be maintained and 

 the necessary resistance furnished for complex move- 

 ments. The food that they eat contains earthy matter 

 which is built in the body into a strong resistant sub- 

 stance firm enough to form a support. The softer parts 

 of the body thus become protected by the development 

 of a skeleton. 



The Skeleton External in Lower Animals. — In lower 

 animals the skeleton is on the outside of the body, but 

 it must not be regarded as a house which the animal 

 enters or leaves as it likes. It is an integral part of the 

 animal. In oysters and clams the body lies inside a shell 

 made of two pieces which may be opened or closed by the 

 activity of strong muscles attached to it. In starfish, 

 lobsters and insects the connection is closer. The en- 

 closing shell is jointed and the muscles are so intimately 

 connected with it that every movement involves a move- 

 ment of the shell. 



