52 THE ANIMALS AND MAN 



The skeleton of a backboneless or invertebrate animal 

 differs from that of a backboned or vertebrate animal (as 

 we have seen in comparing the frog and crayfish) not in the 

 use made of it but in its arrangement and in the part of the 

 body from which it is mainly developed. The skeleton of 

 the invertebrate is developed from the skin, and forms a 

 hard casing over the whole or part of the body (fig. i6). It 

 is therefore called an exo-skeleton. 



In the vertebrates the skeleton is 

 mainly developed from tissues within 

 the body and is called in consequence 

 the endo-skeleton. Even more than 

 in the invertebrates it is a system 

 (fig. 17) of levers, fulcra, and points 

 of attachment for muscles to work 

 with, and is as important a part of the Fig. 17. Skeleton of arm 

 organs of motion as is the muscular °^ ™f "' '^''J' ""'f^P" 



. , . muscle; to show hosv 



system itself. bones and muscle act 



To illustrate the use of the skeleton as levers. (After Jen- 

 of a vertebrate we may examine the "^'"^'^ 

 bones of the hind legs of a cat (fig. 18). The upper bone, 

 the femur, is attached by a joint to the large irregularly 



shaped bone called 

 the ilium, which is 

 firmly bound to the 

 backbone. Below the 

 femur are two bones, 

 the largest, called 

 tibia, being bound 

 by a joint to the 

 Fig. 18. Skeleton of cat. (After Reighard and femur. Below the 



Jennings.) tibia is a group 



of bones, the tarsal bones, pretty firmly fastened together. 

 The largest makes a joint with the tibia. Each of the 

 four tarsal bones toward the toes makes a joint with a 



