VERTEBRATA. 
Cuass I. Pisces. Fisurs. 
Tue animals of this class are distinguished from those already examined, 
by their countless numbers, their varied shapes, their brilliant colors, and 
especially by their economical value. Destined by nature to inhabit and 
people the water, ail their structures and functions tend to thisend. Their 
most general characteristics lie in the possession of cold red blood, breathing 
by gills instead of lungs, a bicamerated or two- chambered heart, fins as 
organs of progression, and a skin either naked or covered with scales of 
varied structure. 
To consider these pile a more closely, the fins consist of a deli- 
cate membrane investing a series of bony or cartilaginous rays, projecting 
from the body along the median line, and from the four homologues of the 
extremities of the terrestrial vertebrata. They have received names derived 
from their situation upon the body. The dorsal fin is on the median line 
of the back, usually single, sometimes sub-divided into two or three fins, of 
various degrees of contiguity. The caudal fin terminates the vertebral 
column in the median line, and is situated ina vertical plane ; the true fishes 
differing in this respect from the fish-like mammalia, the caudal fin in the 
latter being placed horizontally. The third median single fin is the anal, 
situated anteriorly to the caudal, on the anterior median line. This also is 
sometimes divided into two or more portions. The remaining four fins, 
two pectoral and two ventral, situated in pairs, are the homologues of the 
anterior and posterior extremities of the other vertebrata. Their relative 
positions may vary, but they are always found rather on the inferior surface, 
between the anal fin and the head. ‘The pectoral fins are always situated 
just behind the head, and are articulated directly to the skull. The ventrals 
may be entirely posterior to the pectorals, exactly inferior to them, or 
entirely anterior and under the throat. The fins serve as organs of motion. 
and to sustain the fish in an upright position. The principal instrument of 
motion is the caudal fin, which, by its rapid and vigorous strokes from one 
side to the other, causes the animal to move forwards in a straight line, the 
resultant of this lateral flexion. The median fins serve to balance the fish ; 
the pectorals and ventrals, although to a certain extent instruments of 
motion, yet act almost entirely in balancing the fish, and diverting its course 
to the right or left, as also to regulate the rising and sinking in the water. 
Sometimes the rays of several of the fins are thickened into regular spines, 
retaining, to a greater or less extent, the proper integument. Fins without 
distinguishable rays, or where the rays are enveloped in a mass of fatty 
ICONOGRAFHIC ENCYCLOPADIA.— VOL. II. 26 401 
