196 VERTEBRATA 



The African mudfish buries itself in. the mud for several 

 months during the dry season. 



The members of the class Pisces are distinguished from 

 all other vertebrates by the absence of limbs, the presence 

 of gills throughout their entire life, and a simple heart con- 

 sisting of only one auricle and one ventricle. The fish, 

 amphibians, and reptiles are often referred to as the cold- 

 blooded animals and the other two classes, birds and 

 mammals, as warm-blooded animals. This distinction is 

 due to the fact that the two higher classes maintain a con- 

 stant temperature of from ninety-eight to one hundred 

 and five degrees Fahrenheit whether the surrounding at- 

 mosphere be hot or cold. A fish, frog, or snake varies its 

 temperature in accordance with that of the atmosphere. 

 In winter they may have a temperature near the freezing 

 point, while in summer it may be almost a hundred degrees. 



The air bladder, present in all forms except the round- 

 mouthed fish, flatfish, sharks, and rays, lies on the dorsal 

 side of the body cavity above the alimentary canal and 

 in some, such as the herring and trout, opens by a duct 

 into the pharynx. Usually, however, the contained gas 

 is given off through the walls. Its function is to aid the 

 fish in maintaining its position in the water. 



To understand the parts of a fish one of the smaller 

 species, such as the catfish, sunfish, or porgy, should be 

 dissected after the external features have been noted. An 

 eye lies in a deep socket on either side of the head, and 

 is capable of rotation in several directions by means of 

 muscles. No eyebrows or eyelids are present, as in higher 

 forms. The nostrils are two pairs of small openings near the 

 tip of the nose. They lead into the nasal cavity over which 



