CHAPTER LIV 



TERRESTRIAL VERTEBRATES 



The fourth class of the vertebrates, Amphibia, exhibits a transition 

 from hfe in water, which has been characteristic of all vertebrates pre- 

 viously studied, to life in air. Nevertheless, the amphibians do not 

 show a complete emancipation from aquatic life. Some of them remain 



Duct Pigment cells 



Epider- 

 mis 



Mucous 

 gland (b> 



Dermis ■' 



Mucous 

 gland (o) 



Mucous 

 1^ gland (c) 



Pigment cell 



Fig. 244. — Somewhat diagrammatic section of the skin of a frog to show stages in 

 activity of mucous glands. Gland a, the section of which does not go through the duct, 

 is not active; gland h is beginning to form mucus in the epithelial cells which line it; and 

 gland c is actively secreting. 



gill breathers through life and are confined to bodies of water. The 

 majority pass through their earlier stages in the water, then acquire 

 lungs and become air-breathing and terrestrial. A very few have by 

 the acquirement of special adaptations become quite independent of 

 water for their reproduction, but even these require some moisture 

 and cannot live under arid conditions. 



390. Changes Incident to the Acquirement of a Terrestrial Mode 

 of Life. — The first change involves the acquisition of an abundance 

 of mucous glands in the skin to keep it from drying (Fig. 244). A fish 

 possesses glands in its soft epidermis, the secretions of which protect 

 it from contact with the water and from the entrance of infectious 

 organisms, but these are inadequate to prevent rapid drying when the 

 animal is exposed to the air. Even the least adapted amphibian can 

 remain out of water a much longer time than can a fish. The soft 

 skin which salamanders and frogs possess is in toads replaced by a 

 dry, hard skin, which adjusts them to relatively drier situations. 



359 



