284 



ELEMENTARY ZOOLOGY. 



Moving in the water as the fish does he has four prob- 

 lems: propulsion; horizontal steering; vertical steering; 

 and keeping himself upright in the water. The tail is 

 the organ that controls the first. Difference in the 

 strength of the right and left strokes of the tail will turn 

 the animal to right or left; this would be aided by the 

 action of the small fins. The paired fins can be used to 

 steer up or down. Probably the air-bladder, a long 



FIG. 109. Diagram of vertebrae of a bony fish. A, caudal; B, trunk; c, centrum or 

 body of vertebra; cAi.notochord; h.a, haemal (or visceral) arch surrounding haemal cavity 

 (h.c.) ; n.a., neural arch, surrounding neural (nervous) cavity (n.c.) ; r, rib. 



Questions on the Figure. In life, what occupies the neural cavity ? 

 What occupies the haemal cavity in the tail region? In the trunk 

 region ? 



internal organ filled with gas, assists in this. If the 

 muscles of the posterior part of the body wall are con- 

 tracted, the air in the bladder will be pushed forward 

 and this end will be of less specific gravity than the 

 posterior end, and will be pointed slightly upward as the 

 animal swims. The reverse would be accomplished by 

 contracting the anterior body muscles. This would aid 



