* 



64 INTRODUCTION. 



pie, and has no epiglottis &c., neither is there a complete dia- 

 phragm between the breast and the abdomen. 



The ovipari are divided according to their mode of respira- 

 tion, their temperature, the atmosphere they inhabit, their mo- 

 tions, the appendages of the skin &c. into three classes: Fishes, 

 reptiles and birds. 



58. Fishes are evidently organized for natation; they are 

 suspended in a fluid nearly as heavy as themselves. Many, 

 under the vertebral column, have a bladder filled with air, 

 which by its own contraction or dilatation varies the specific 

 gravity of the animal. The head variable in its form, is of a 

 very complicated structure, as regards the cranium, the jaws, 

 and the distribution of the teeth. The limbs are abbreviated 

 and formed into fins; other fins occupy the back, the top of the 

 tail and its extremity. The number of members varies, most 

 generally there are four, sometimes only two, and in some 

 they are totally wanting. Their position and connexion with 

 the trunk also vary greatly, so do the organs of digestion; the 

 pancreas is generally superseded by intestinal appendages. 

 The circulation is double, i. e. all the blood passes through the 

 respiratory organs, but the atmosphere respired is aerated wa- 

 ter: for this purpose, they have on the sides of the neck, an 

 apparatus of organs called branchiae. These are leaflets attach- 

 ed to little lateral arches of the os hyoides, and composed of 

 numerous membranous laminas, covered by a net-work of numer- 

 ous blood-vessels; this opening, is, besides, furnished with a 

 branchial membrane, supported by the processes of the hoyoi- 

 deus and a bony operculum. The water which the fish com- 

 presses in its mouth as if to swallow it, escapes between the 

 divisions of the branchiae and acts on the blood. The heart has 

 only one auricle, which receives the veins of the body, and one 

 branchial ventricle. The blood, after having traversed the 

 branchiae, is directed into a large vessel under the spine, which, 

 exercising the functions of ventricle and aorta, transmits it to 

 every part of the body. 



Fishes have elongated kidneys stretched along the sides of 

 the spine and a bladder. Their testicles are two enormous 

 glands, generally known by the name of milts, (laite) their 



