380 



COLLEGE ZOOLOGY 



Locomotor organs 



The body of the perch and of most other 

 fishes is streamHned and offers Httle resist- 

 ance to the water through which the animal 

 swims. By means of an air bladder, it is 

 kept at the same weight as that of the water 

 it displaces. The fish is thus able to remain 

 stationary without much 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, thus enabling the fish 



to swim. Similar movements are employed 

 in sculling a boat, when one oar at the stern 

 is moved from side to side. 



The fins are integumentary expansions 

 supported by bony or cartilaginous rays. The 

 paired lateral fins (pectoral and pelvic) are 

 used as oars in swimming when the fish is 

 moving slowly. They also aid the caudal fin 

 in steering. Movement up or down results 

 from holding the lateral fins in certain posi- 

 tions—obliquely backward with the anterior 

 edge higher for the ascent, and obliquely 

 forward for the descent. 



Fishes must maintain their equilibrium 



Lateral line 

 Operculum 

 Exfernal nares 

 Premox 



MandibI 

 Maxilla 



Spinous dorsal fin 



Soft dorsal fin 



i 



Branchiostega! membrane 

 Pectoral fin 



Pelvic fin 



Anal fin 



Ml 



Caudal fin 



Figure 253. External features of a typical bony fish, the yellow perch. 



in some way, since the back is the heaviest 

 part of the body and tends to turn them 

 over. The dorsal, anal, and caudal fins in- 

 crease the vertical surface of the body, and, 

 like the keel of a boat, assist the animal in 

 maintaining an upright position. The paired 

 lateral fins are also organs of equilibrium, 

 acting as balancers. However, experimenta- 

 tion shows that if one or two of the paired 

 fins are removed, the fish soon learns to com- 

 pensate for their loss. 



Skeletal system 



The exoskeleton of the perch includes 

 scales and fin rays. The scales develop in 

 pouches in the dermis. They are arranged 

 in oblique rows and overlap like the shingles 

 on the roof of a house, thus forming an ef- 

 ficient protective covering. The posterior 

 edge of each scale which extends out from 



under the preceding scale is toothed and 

 therefore rough to the touch. Scales of this 

 kind are called ctenoid scales (Fig. 261). 

 The fin rays support the fins. Those of the 

 spinous dorsal fin (Fig. 254) and of the 

 anterior edge of the anal and pelvic fins are 

 unjointed and unbranched spines. The 

 caudal, pectoral, pelvic, soft dorsal, and 

 anal fins are supplied with jointed, and, usu- 

 ally branched, soft fin rays. 



The bones of the endoskeleton are shown 

 in Fig. 254. They include the skull, vertebral 

 column, ribs, pectoral girdle, and the inter- 

 spinous bones which aid in supporting the 

 unpaired fins. The body of the fish is, to a 

 considerable extent, supported by the sur- 

 rounding water; consequently, the bones do 

 not need to be so strong as those of land 

 animals, like birds and mammals, which 

 support the entire weight of the body. 



The vertebrae are simple and compara- 



