434 



COLLEGE ZOOLOGY 



animal, for, although the course is altered largely by the pointing 

 of the head and tail in the desired direction, the lateral fins assist 

 in swerving the body to one side or the other, either by executing 

 more powerful strokes on one side, or by the expansion of one 

 fin and the folding back of the other. These methods are like 

 those used in steering a rowboat with 

 oars. Movement up or down results 

 from holding the lateral fins in certain 

 positions obliquely backwards with the 

 anterior edge higher for the ascent, and 

 obliquely forwards for the descent. 



Fishes must maintain their equilibrium 

 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 

 increase the vertical surface of the body 

 (Fig. 369) and, like the keel of a boat, 

 assist the animal in maintaining an upright 

 position. The paired lateral fins are also 

 organs of equilibration, acting as balancers; 

 if both pectoral fins are removed, the an- 

 terior end of the fish sinks downward ; if a 

 pectoral or both pectoral and ventral fins 

 are removed from one side, the fish turns 

 toward that side; and if all four lateral 

 fins are cut off, the fish turns completely 

 over with the ventral surface upward. 



The Skeleton. The exoskeldon 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 efficient 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. 371, A). The 



FIG. 370. Diagram 

 to illustrate the mode in 

 which the tail of an or- 

 dinary fish is used in 

 swimming. (From the 

 Cambridge Natural His- 

 tory, after Pettigrew.) 



