THE FROG AND VERTEBRATES IN GENERAL 167 



coloration ; the ability to change the color scheme so as to blend 

 as closely as possible with the surroundings — an animal camou- 

 flage, as it were. In the Frog, color changes take place by the 

 expansion and contraction of specialized pigment cells, or chro- 

 matophores, which lie just below the surface. 



The wet, mucus-covered skin of the Frog undoubtedly plays a 

 large role in the respiratory activities. It is plentifully supplied 

 with blood vessels, and the interchange of carbon dioxide and 

 oxygen through the skin is a continuous process. The small 

 poorly-developed lungs of the Frog are not able to handle the 

 entire respiratory exchange, and the animal will die finally if the 

 exchange through the skin is entirely stopped, as happens when 

 it remains dry for a prolonged period. 



Finally, in the Frog, and other types, the skin is abundantly 

 supplied with secreting structures, or glands. Two varieties, the 

 mucus and the poison glands, are usually found. The mucus 

 glands continually secrete a slimy material which covers the skin 

 and protects against drying when the Frog is out of water. The 

 secretion of the poison glands is known to be of a poisonous nature 

 and is undoubtedly protective. 



B. The Body Tissues 

 Before proceeding to a description of the various organ systems 

 of the Vertebrate, attention should be called to the epithelial, 

 muscular, and supporting tissues which, together with the 

 nervous and vascular elements, are almost everywhere present 

 in all the organs of the body. Thus, to take a single example, the 

 alimentary tract — in addition to the all-important mucosa cells 

 which actually carry on the essential functions of digestion and 

 absorption — contains epithelial, muscular, connective, vascular, 

 and nervous tissues. Organs, then, are composed of a variety 

 of tissues. Consideration is given to the vascular and nervous 

 tissues in later sections, so we may now give attention to the 

 epithelial, muscular, and supporting elements. (W. pp. 132-145.) 



1. Epithelial Tissue 

 Epithelial cells cover the exposed surfaces of the body structures 

 both externally and internally. Thus the outermost portion of 

 the skin, which is in contact with the environment, is covered with 

 several layers of epithelial cells, the most external of which are 

 exceedingly thin, flattened cells (squamous epithelium) which 



