718 pisjes. 



having their bodies admirably constructed for the liquid in which they 

 move, have in addition a singular apparatus, for rendering them specifi- 

 cally lighter or heavier than water. This consists in a membranous sac, 

 containing air, called the air-vessel, or swimming bladder, which is placed 

 under the spine, and by compressing or dilating which, they are supposed to 

 rise or sink in the water. This vessel forms two compartments in the carp, 

 and often communicates with the intestines in fishes which inhabit lakes or 

 rivers. One of the chief differences in structure, which characterizes the 

 present class of animals, is their respiratory apparatus. Living in a fluid 

 element, their respiration is necessarily adapted to the nature of that fluid. 

 This is accomplished by means of an apparatus named gills, or bronchiae 

 placed on the sides of the neck or head. These bronchia? consist of numer 

 ous lamina?, suspended on arches attached to the hyoid bone, each composed 

 of a great number of separate laminae, covered with a tissue of innumerable 

 blood-vessels. The water which they swallow passes between these 

 laminae, and escapes by the bronchial openings. In its passage, the blood 

 which is sent from the heart to the gills is acted upon by the air contained 

 in the water. After undergoing this change, it is collected in an arterial 

 trunk under the spine, which, though resembling, in anatomical situation, 

 the aorta of animals with a double heart, performs the functions of a left ven- 

 tricle, and distributes it by numerous ramifications through the body, from 

 which it returns to the heart by the veins. The bronchial openings are 

 covered either by an osseous moveable plate, which is termed the gill cover, 

 or operculum, or, by a simple membrane, with one or more openings. 



The whole of blood in fishes is sent by the heart to the bronchial vessels, 

 and is then venous, or dark blood ; but when it has been exposed to the air 

 in the water, it assumes the arterial, or red color, and passes into other ves- 

 sels, which unite in the great arterial trunk under the spine. Thus the 

 neart has but a single auricle, a single ventricle, and a single artery; and it is 

 believed that the little portion of heat developed in this mode of respiration, 

 is owing to the small quantity of air to which the blood is at one time 

 exposed, in passing through the bronchia?. 



The vertebra? of fishes are united together by concave surfaces, filled with 

 cartilage. In the greater number, these vertebra? have long spinous pro- 

 cesses, which keep :he body in a vertical position. The ribs are often joined 

 to transverse processes. Though the head in fishes varies more in point 

 of form, than in any other class of animals, it always consists of the 

 same number of bones. The frontal bone is composed of six pieces ; the 

 parietal, of three ; the occipital, of five ; the sphenoidal bone of five, and 

 each temporal bone of two pieces. The cranium forms but a small portion 

 of the head. The brain is enveloped by gelatinous matter, and forms many 

 ganglions or consecutive knots, as in the reptiles; and there are ganglions or 

 snots, besides, at the base cf the olfactory nerve. A superficial nerve also 

 runs along the body, almost immediately under what is called the lateral 



