PISCES 363 



many extinct forms the external covering was made up of large 

 plates fused into a dense armor. 



382. 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, 180, q). 



383. 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 how- 

 ever more difficult to penetrate. The specific gravity of the 

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

 it varies within narrow limits. Four problems are thus pre- 

 sented 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, especially in slow motion. 



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 semicircular canals 

 probably assist the animal in appreciating changes in its posi- 

 tion, its orientation, thus enabling it to choose its direction. 



4. Balancing. Since most fishes are flattened from right to 

 left there is some difficulty in keeping an upright position. The 

 sense of position is doubtless given by the semicircular canals. 

 The paired fins are used in balancing, as are the dorsal and anal. 

 Doubtless all the fins along the body of the fish are used some- 

 what as the keel of a boat or the planes of an air ship or a 

 submarine. 



