THE MOTIONS OF ANIMALS 69 



and all of the above-described kinds of motion are found 

 in some of these. But motion is produced mainly by the 

 contraction of fibers, which are grouped into masses called 

 muscles. In the higher groups of animals the great 

 mass of the body consists of muscles composed of con- 

 tractile fibers, which are to be thought of as modifications 

 of a fiber such as exists in the stalk of the Vorticella, one 

 of the smallest and simplest of living animals. It is as if 

 the contractile substance had appeared under different 

 forms, but having proved most useful in the form of fibers 

 as animals rose higher in the scale this form became very 

 extensively used. 



In such forms as the hydroids and jellyfishes the con- 

 tractile fibers are not gathered together into such definite 

 muscles as in the higher animals, nor are there such well- 

 developed firm body-parts for the fibers to pull against. 

 These contractile fibers are only extensions of- certain 

 body-cells, as in Hydra (see Chapter IX), or form a more 

 or less scattered net-work, gathered into ill-defined bands 

 or sheets, as in the sea-anemones and jellyfishes. From 

 the starfishes and worms, through the crabs and insects, 

 the muscular parts become more and more definite in 

 form and prompt in action, and there appears more and 

 more perfect development of firm parts. Some of these 

 serve as points of attachment from which the muscles can 

 pull, and some act as levers to make a push on the 

 external water, earth, or air. The swift, strong, and 

 accurate motions of the insects and of the backboned 

 animals require rigid levers, firm points for levers to work 

 against, or fulcra, and other firm points for the attach- 

 ment of muscles from which to exert their pull. These 

 firm solid parts, the levers, fulcra, and attachment points 

 of muscles, constitute the skeleton of the animal body. 



The skeleton of a backboneless or invertebrate animal 

 differs from that of a backboned or vertebrate one not in 



