256 



SKELETONS OF THE LOWEE, ANIMALS 



the most perfect type of external skeleton found. In 

 these animals the skeleton is formed of a horny material 

 called chitin, which completely incloses all parts of the 

 body, except the apertures for the mouth and othdr open- 



FlQ. 106 — External jointed skeleton (lobster). 



ings. By making this secretion less stiff at points, very 

 perfect joints are formed which allow a great freedom of 

 movement to the legs and body. The arthropods are, 

 in fact, animals encased in a suit of excellently jointed 

 armor. This form of external skeleton, while excellent as 

 a protection, and well adapted to movement, necessarily 

 restricts the growth of the animal. In other words, when 

 the crustacean increases in size he is in the same situation 

 as the small boy in a tight suit of clothes, and the result is 

 the same, he bursts the clothes and must have a new and 

 larger suit suppUed. The manner in which this is done 

 is by withdrawing the body from the old, broken covering, 

 and after expansion, allowing the new one to form from 



