360 ZOOLOGY. 



of scales altogether, the skin of such often being supplied with 

 numerous mucous glands. In many extinct forms the external 

 covering was made up of large plates fused into a dense armor. 



375. The Skull. The skull in fishes is especially note- 

 worthy for the looseness of the connection between the facial 

 bones (i. e., the visceral or branchial arches) and the cranium. 

 They are readily separated from the cranium. The lower 

 jaw is not articulated directly with the brain-case but with the 

 upper jaw (see Fig. 177, q). 



376. Locomotion. Fishes are aquatic and are complete 

 masters of their medium. The density of water as compared 

 with air makes the matter of support in the medium much 

 easier for the fish than for the bird. The denser medium is 

 however more difficult to penetrate. The specific gravity of 

 the fish as a whole does not differ widely from that of water, 

 although it varies within narrow limits. Three problems are 

 thus presented to the fish for solution : 



1. The Regulation of Specific Gravity. This is effected in 

 part at least by the air bladder. The body muscles may bring 

 about the compression of the contained gas and thus decrease 

 the size without change of weight. 



2. Propulsion. The chief organ of propulsion is the caudal 

 fin, acted on by the powerful lateral muscles of the body. The 

 resultant of the alternate strokes against the water is forward 

 motion. This may be supplemented by the action of the paired 

 fins. 



3. Steering. This is accomplished in part by the changes 

 in specific gravity and the regulation of the stroke of the tail, 

 and in part by the action of the paired fins. The semi-circular 

 canals probably assist the animal in appreciating changes in 

 its position, its orientation, thus enabling it to choose its 

 direction. 



377. Supplementary Exercise for the Library. What is the structure 

 and position of the " swim-" or air-bladder in fishes ? With what organ 

 is it related? Does it communicate with the outside? Are there any evi- 

 dences that it is of value as n respiratory organ in any of the fishes? 



