THE FISH. 



to swimming as that of birds is for flight. Suspended 

 in a fluid nearly as heavy as themselves, the former 

 have no need of wings to sustain them in it. A great 

 number of species have immediately under the spine 

 an air vessel, which by compression or dilatation 

 changes the specific gravity of the fish, and assists it 

 in rising or descending in the water. Progression is 

 executed by the motion of the tail, which strikes the 

 water alternately right and left, and forces the fish 

 forward ; the gills discharging the water backward 

 probably contribute also thereto. Hence, the limbs 

 being but of little use are greatly reduced ; the pieces 

 analogous to the bones- of the arms and of the legs 

 are extremely short or entirely concealed ; rays more 

 or less numerous, supporting membranaceous fins in 

 some degree represent fingers and toes. The fins 

 corresponding to the anterior extremities are said to 

 be pectoral, those which answer to the posterior are 

 said to be ventral. Other rays attached to particular 

 bones placed upon or between the extremities of the 

 spinous processes support vertical fins on the back, 

 under the tail, and at its extremity, which by being 

 erected or depressed, extend or diminish at the will of 

 the fish, the surface which strikes the water. The 

 upper fins are said to be dorsal, the lower anal, and 

 that of the end of the tail caudal. The rays are of 

 two sorts ; the one consisting of a single bony piece, 

 generally hard and pointed, but sometimes flexible 

 and elastic, divided longitudinally, these are called 

 spinoffs rays ; the others, which are composed of a 



