CLASS PISCES 



433 



eastern United States, and ranges west to the Mississippi Valley. 

 Its body is about a foot long and is divisible into head, trunk, and 

 tail. There are two dorsal fins, a caudal fin, a single median anal 

 fin just posterior to the anus, two lateral ventral fins, and two 

 lateral pectoral fins. On each side of the body is a lateral line. 

 The head bears a mouth with well-developed jaws armed with 

 teeth, a pair of lateral eyes, a pair of nasal apertures in front of 

 each eye, and gill-covers or opercula beneath which are the gills. 

 The skin is provided with a number of dermal scales which are 

 arranged like the shingles on the roof of a 

 house, and protect the fish from mechanical 

 injury. 



Locomotor Organs. — The body of the 

 perch, and of most other fishes, is spindle- 

 shaped and offers little resistance to the 

 water through which the animal swims 

 (Fig. 369). It is kept at the same weight 

 as the amount of water it displace^ by 

 means of an air-bladder. The fish is thus 

 able to remain stationary without muscular 

 exertion. The principal locomotor organ is 

 the tail. By alternating contractions of 

 the muscular bands on the sides of the 

 trunk and tail, the tail with its caudal fin is 

 lashed from one side to the other, moving in a curve shaped like 

 a figure 8 as shown in Figure 370. Similar movements are em- 

 ployed in sculling a boat, and the method of progress is analogous 

 to the action of the screw of a steamer. During the flexions 

 and extensions of the tail, the trunk is curved in such a way as 

 to bring about the most effective extension or forward stroke 

 and a weak flexion or non-effective stroke. 



The fins are integumentary expansions supported by bony' or 

 cartilaginous rays. The paired lateral fins (pectoral and ven- 

 tral) are used as oars in swimming, but only when the fish is 

 moving slowly. They also aid the caudal fin in steering the 

 2 F 



FlG. 369. — Front 

 view of a fish (Spanish 

 mackerel). (From 

 Dean.) 



