176 



BIOLOGY 



The whole body of the frog is covered with a smooth skin, 

 which is always moist and is abundantly supplied with blood 

 vessels, especially under the arms and on the side of the body. 

 The skin is everywhere loosely attached to the underlying 

 flesh and in certain rather large areas is not attached at all, 

 large spaces being thus left between 

 it and the flesh. These are lymph 

 spaces and are filled with a clear liquid 

 called lymph. When the skin is ex- 

 amined microscopically, it is found to 

 be made of two layers; Fig. 87. The 

 outer layer, the epidermis, ep, is thin, 

 while the inner layer, the dermis, d, is 

 quite thick. The epidermis is made 

 of several layers; the cells of the inner 

 layers are large, rounded, growing cells, 

 while the outer ones are flattened and 

 lifeless. The epidermis increases in 

 thickness from its inner side, and is 

 constantly wearing away on its outer 

 side. The dermis is a mass of con- 

 nective tissue fibers, among which lie 

 glands, blood vessels, nerves, and 



numerous pigment (Lat. pingere = to paint) cells which give 

 the color to the skin. 



The Skeleton. The frog has an internal bony skeleton. 

 An internal skeleton is the most distinctive characteristic 

 of the highest animals. Animals with such a skeleton are 

 called vertebrates, a group comprising fishes, amphibians, 

 reptiles, birds, and mammals. No other animals except verte- 

 brates possess true bones. This bony skeleton gives support 

 to the softer parts, gives form to the body, serves to attach 

 the muscles, and enables them to produce the movements of the 

 animal. The skeleton is made of about ninety articulated bones, 

 i. e. t united together at the joints. Some of these form mov- 



FIG. 87. SECTION 

 THROUGH THE SKIN OF 



THE FROG 



ep, epidermis; 

 d, dermis. 



(Modified from Howes.) 



