198 PISCES. 



as that of the Bird for flight. Suspended in a liquid of nearly the 

 same specific gravity as its own body, there was no necessity for 

 large wings to support it. In a great number of species, immedi- 

 ately under the spine there is a bladder filled with air, which, by 

 compression or dilatation, varies the specific gravity of the fish and 

 assists it to rise or descend. Progression is effected by the motions 

 of the tail, which, by striking the water alternately right and left, 

 forces them forward; the branchiae, by impelhng the water back- 

 wards, may also contribute to this effect. The limbs being thus of 

 but little use, are greatly reduced; the parts analogous to the bones 

 of the arms and legs are extremely short, or even completely con- 

 cealed; rays, more or less numerous, which support membranous 

 fins, form a rude representation of the fingers and toes. The fins 

 which correspond to the anterior extremities are termed pectorals, 

 and those which answer to the posterior ones, ventrals. Other rays 

 attached to particular bones placed on or between the extremities 

 of the spinous apophyses support vertical fins on the back, under the 

 tail, and at its extremity, which, by being raised or lowered, increase 

 or diminish the surface which strikes against the water. The su- 

 perior fins are called dorsal, the inferior anal, and that at the end of 

 the tail caudal. The rays are of two kinds; some of them consist 

 of a single bony piece, usually hard and pointed, sometimes flexible 

 and elastic, divided longitudinally — these are called spinous rays; 

 others are composed of a great number of small articulations, and 

 generally divided into branches at their extremity — they are the 

 soft, articulated, or branched rays. 



There is as much variety among Fishes, with respect to the number 

 of limbs, as among Reptiles. Most generally there are four; some 

 have but two, and in others they are totally wanting. The bone 

 which is analogous to the scapula, is sometimes held among the 

 muscles as in the higher animals, and at others is attached to the 

 spine, but most commonly it is suspended on the cranium. The 

 pelvis rarely adheres to the spine, and very frequently, instead of 

 being behind the abdomen, is before it, and connected with the hu- 

 meral apparatus. 



Besides the usual parts of the brain which are arranged as in 

 Reptiles one after the other, Fishes have knots or ganglions at the 

 base of their olfactory nerves. 



Their nostrils are simple cavities at the end of the muzzle almost 



