214 



FISHES 



cavity of the perch there is a, long, tllill-^^'alle(l sac, the air 

 bladder. It is fihed \\'itli air and is evidently an organ for 

 regulating tlie position of tlie fisli in the \\'ater. 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 lighter, and it rises. 

 In some fishes that rest most of the time 

 on the bottom there is no air bladder. 

 In other fishes it seives as a lung. 



The posterior jjart of a fisli is flexible 

 and has a consecjuent freedom of motion 

 not found in the more rigid anterior 

 part. The tail and tail fin constitute 

 the principal organ of locoiriotion. A 

 civiick side\^'ise stroke ■\\'ith the tail in one 

 direction followed instantly l))' another 

 stroke in the opposite direction forces 

 the bod}^ forward in a straight line. 

 The physical princi]iles of this motion 

 Fig. 1.50. — Diagram are sho\Mi in Fig. 150. In the move- 



iUustratiiig (lie loco- i r il C 1 il • 1 c l 



motion of a fish. The ^ent of the fish the pau'cd fins are used 

 tail dcsf-ribes the arc to balance the bodv and to ascend or 

 of an ellipse .there- 'lesccnd in the water. The dorsal fins 



sultantoi the two im- 

 pulses is tiic straigiit guidc and steady the boily in its progress. 



""<" ■" f™"*- The tail fin is also used to guide the fish. 



Reproduction and development. — The sexes are separate. 



The ovai'ies of the female lie afiove the intestine and open 



through the oviducts just behind the anal opening. The 



spermariesof the male commmiicate with the outside through 



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



eggs are extruded from the body and are fertilized b,v the 



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



