214 



FISHES 



cavity of the perch there is a long, thin-walled sac, the air 

 bladder. It is filled with air and is evidently an organ for 

 regulating the position of the fish in the water. The gas in 

 the sac is compressed by the contractions of its muscular 

 walls and the fish sinks. Vice versa, when the gas expands, 

 the bladder becomes larger, the body hghter, and it rises. 

 In some fishes that rest most of the time 

 on the bottom there is no air bladder. 

 In other fishes it serves as a lung. 



The posterior part of a fish is flexible 

 and has a consequent freedom of motion 

 not found in the more rigid anterior 

 part. The tail and tail fin constitute 

 the principal organ of locomotion. A 

 quick sidewise stroke with the tail in one 

 direction followed instantly by another 

 stroke in the opposite direction forces 

 the body forward in a straight line. 

 The physical principles of this motion 

 ■Diagram are shown in Fig. 150. In the move- 



illustrating the loco- i_ c ^-x n i ^^ - -i ^ i 



motion of a fish. The ^ent of the fish the paired fins are used 

 tail describes the arc to balance the body and to ascend or 

 of an ellipse; there- descend in the Water. The dorsal fins 



sultant of the two ua- 



puises is the straight guide and Steady the body in its progress. 



Hne in front. -p]^g ^^^ ^^ Jg g^jg^ ^gg^ ^^ g^j^g ^j^g ^^T^_ 



Reproduction and development. — The sexes are separate. 

 The ovaries of the female he above the intestine and open 

 through the oviducts just behind the anal opening. The 

 spermariesof the male commimicate with the outside through 

 the sperm ducts which open just back of the anus. The 

 eggs are extruded from the body and are fertihzed by the 

 sperms which are set free in the water. The perch does not 



Fig. 150. - 



