VERTEBRATA. 



Class I. Pisces, Fishrs. 



The animals of this class are distinguished from those already examined, 

 by their countless numbers, their varied shapes, their briUiant colors, and 

 especially by their economical value. Destined by nature to inhabit and 

 people the water, all their structures and functions tend to this end. 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 characteristics 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 in a 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 a7ial, 

 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 ENCYCLOP.fiDIA. — VOL, II. 26 401 



