346 



GENERAL PRINCIPLES 



733. The development of an internal skeleton by the Verte- 

 brates called for a new solution of these growth problems. But 

 in the Vertebrates numerous integumentary structures, each 

 with its peculiar method of growth, are also found. We will 

 only consider the epidermis here. This layer of the skin is con- 

 stantly growing in all the terrestrial Vertebrates and its dead, 



Fig. 214. — Ecdysis of the blue crab. The animal (lower part of the figure) 

 has almost freed itself of the shell from which it escapes by backing out. 

 X1/2. 



outer layers are cast either at some intervals of time or as a con- 

 tinuous process. In the Amphibia, this layer is cast at intervals 

 in large or small patches. In Snakes it often comes away in a 

 single piece. In Birds and Mammals the epidermis is constantly 

 shedding in minute scales, but in these two classes there is 

 usually a well-marked period of moulting or shedding of feathers 

 and hair. When the epidermis is cast it is, of course, not the 



