CHAPTER XXVII 

 PISCES, TRUE FISH 



This important class includes quite an extensive variety of dif- 

 ferent forms. They are aquatic and possess the usual adaptation of 

 gills for respiration, and paired fins as well as median fins to assist 

 in locomotion. Most forms within the class have scales as an exo- 

 skeletal covering of the skin. The endoskeleton is primarily bony. 

 Pectoral and pelvic girdles are developed to support the paired fins, 

 but the pelvic girdle is usually small. The fins are supported by 

 fairly well-developed fin rays. The majority of families in this class 

 possess a swim bladder. The typical shape of the fish's body is 

 fusiform or spindle-shaped, with all of the original features of 

 stream-lining. The shape assists in dividing the water as the fish 

 moves through it. As the water passes over the thicker part of the 

 body, it rushes in to push forward on the posterior slopes of the 

 spindle form of the body. This is an adaptation for easy production 

 of speed. The sedentary forms of fish usually tend to lose this shape 

 and become flattened or otherwise modified. The shape of the body 

 varies from that of the long slender eel to that of the globe-shaped 

 box-fish and inflated puffers which can float like balloons. The 

 sea horse is one of a group of very peculiarly shaped forms. Still 

 another peculiar adaptation is the flying fish. The fins of fish are 

 found singly in the form of a dorsal median fin, sometimes divided 

 into two ; a single caudal fin over the tail ; a ventral median anal fin 

 in most species; a pair of pelvic or ventral fins which are quite 

 variable in position and in some forms rudimentary, and the paired 

 pectoral fins. These paired fins are supported by bony girdles. The 

 pelvic fins of the perch are located almost immediately ventral to 

 the pectoral fins, while in the bullhead catfish they are just anterior 

 to the anus. In this catfish there is a second dorsal, which is com- 

 posed entirely of skin and is called an adipose fin. The structure of 

 the caudal fin and posterior end of the vertebral column is distinctive 

 and has been classified. The most primitive type of tail is the 

 diphyceroal in which both the cutaneous and osseous parts are 

 equally divided between dorsal and ventral regions. The hetero- 

 cercal tail is asymmetrical and the tip of the vertebral column ex- 



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