718 PIS SES. 
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 
tise 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 bronchie 
placed on the sides of the neck or head. These bronchiz consist of numer 
ous lamin, suspended on arches attached to the hyoid bone, each composed 
of a great number of separate lamin, covered with a tissue of innumerable 
blood-vessels. The water which they swallow passes between these 
lamine, 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 srterial trunk under the spine. Thus the 
neart has but a single auricle, a single ventricle, and asingle artery; and itis 
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 bronchiz. 
The vertebre of fishes are united together by concave surfaces, filled with 
cartilage. In the greater number, these vertebre have long spinous pro- 
cesses, which keep the body in a vertical positian. The ribs are often joined 
to transverse processes. Though the head in fishes varies more in poit 
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 
nots, besides, at the base ef the olfactory nerve. A superficial nerve also 
runs along the body, almost immediately under what is called the Jateral 
