188 STUDIES IN PHYSIOLOGY 



this means of protection of these crus-ta'cea (Latin crusta = 

 the hard surface of a body), to that afforded by a coat of 

 mail worn by the warriors of the middle ages. This com- 

 parison is more striking since some of the plates of this 

 outside skeleton move over each other at the joints in a 

 manner similar to the iron plates of the armor (Fig. 82). 



Vertebrate Skeletons. It would be impossible to give in a 

 limited space any extended account of the skeletons of the 



FIG. 82. The Crayfish, showing Hard Exo-skeleton. 



various groups of vertebrates. A few facts may be noted, 

 however. 



We have learned that the skeleton of a young child is 

 first formed of cartilage; some of the fishes (sharks and 

 sturgeon) possess a cartilaginous skeleton throughout life. 

 In all of the vertebrates, with the exception of the snakes 

 (and a few rare lizards and amphibia), we find anterior ap- 

 pendages that correspond to the arms of man, and posterior 

 appendages corresponding to legs. Both sets of these 

 appendages are used by fishes in swimming; the posterior 

 limbs of a frog are employed in jumping and swimming; 

 while in birds the anterior appendages are of use in flying. 



