STRUCTURE OF FISHES. 



side, and with a middle prolongation of the hyoid apparatus, and 

 is attached to the cranium through the medium of two bones, 

 which Cuvier considers analogous to the scapula; finally, the 

 stylet which arises from it, and is prolonged backwards upon the 

 ribs, is ordinarily formed of two pieces, and may be compared 

 to a coracoid bone. 



14. The posterior extremity is less complicated; the rays or 

 the ventral fin are supported by a single bone, generally trian- 

 gular, which often becomes attached in front, to the middle 

 junction of the bony belt of tire pectoral extremity, and at other 

 times it is merely suspended in the flesh. 



15 In cartilaginous fishes, the arrangement of the skeleton 

 differs from what has just been described. The head especially, 

 is much more simple in its structure. 



16. The muscular apparatus is composed of muscles destined 

 to flex the vertebral column laterally, and also to move the tail; 

 they form the largest part of the mass of the body of fishes. 

 By striking the water laterally, by alternate flexions of the trunk 

 and tail, these animals communicate to their body, nearly the 

 whole of the rapidity they have in swimming. Their vertical 

 fins serve to increase the extent of the species of keel or oar 

 they form, while the chief use of the pectoral and ventral fins, 

 in general, is to influence the direction of their course, and to 

 maintain the equilibrium of the animal. 



17. A peculiarity of their organization, which is of great assis- 

 tance in swimming, is the existence of a sort of pouch filled with 

 air, and so placed that it can be compressed at will. This swim- 

 ming, or air-bladder, which is placed in the abdomen beneath the 

 dorsal spine, ordinarily communicates with the oesophagus, or 

 stomach, by a canal, through which the air contained in it, may 

 escape ; but this fluid does not seem to enter by that route ; it is 

 produced by secretion, the seat of which is in a portion of the 

 parietes of the reservoir itself, which is of a glandular structure. 

 By the motions of the ribs, this bladder is more or less com- 

 pressed, and, according to its volume, it gives to the body of the 

 fish, a specific gravity, equal, superior or inferior to that of the 

 water, and causes it thus to remain in equilibrium, to descend, or 

 ascend in this liquid. It is remarked that it is often wanting, 

 and that it is very small in those species that swim near the 

 bottom, or bury themselves in the mud. 



14. What is the character of the posterior extremity ? 



15. Is the skeleton the same in all fishes 1 



16. By what means do fishes move? What is the use of the fins? 



17. What is the air bladder in fishes? What is the source of the air 

 contained in it ? 



