420 
VERTEBRATA. 
by tlieir taking the water in at the mouth and expelling it from under the gill-covers. The 
blood, which is constantly sent to the branchiae from the heart, is distributed by means of the 
arteries to every part of the body, Avhence it returns to the heart by means of the veins. 
As the breathing apparatus in the fish is suited to aquatic habits, so likewise is every part of 
its structure. The body is generally of an elongate, oval, compressed form, covered with scales 
directed backward, and furnished with fins, thus being beautifully adapted for swimming. Many 
fishes, moreover, have an air-bladder filled with air, situated immediately beneath the spine, by 
the dilatation or compression of which their specific gravity is said to be varied. The thoracic 
part of the body is thrown forward toward the head, so that the fishes may be said to have no 
neck, and thus the hinder part of the body is more free and fitted for motion. The limbs are 
formed into fins, the fore-legs constituting Avhat are termed the pectoral fins, and the posterior 
extremities the ventral ; besides these fins, ordinary fishes are furnished with one or two dorsal 
fins, an anal fin, and a caudal fin, or tail. 
SKELETON OF THE PERCH. 
All these fins are not always present, nor when present are they always in the same relative 
positions ; both the absence of certain fins, and the peculiar position of these organs, afford 
characters in the classification of fishes. The fins consist of a thin elastic membrane supported 
by rays. The rays are of two kinds : those which consist of a single bony piece, usually hard 
and pointed, are termed spinous rays ; and when they are formed of numerous portions of 
bone united by articulations, and frequently divided longitudinally into several filaments, they 
are called fiexiUe rays. The principal organ of motion is the tail ; the dorsal and ventral fins 
apparently serve to balance the fish, and the pectorals to arrest its progress when required. 
The bones of fishes are of a less dense and compact nature than in the higher order of animals ; 
in some, indeed, they are wholly cartilaginous. The skeleton may in general be divided into 
four chief parts — the Vertebral Column, the Head, the Respiratory Apparatus, and the Limbs. The 
Vertebral Column consists of vertebraB which are concave at each end and pierced in the middle ; 
and when joined together the hollow space between each two is occupied by a gelatinous sub- 
stance, which passes from one space to the next through the hole in each bone. This hole is 
usually very small, but in some it is so large that the bones of the vertebrae are mere rings. 
To the vertebrae are attached the Ribs ; in fact the ribs are the main support of all the other 
bones. The Head varies more in form than in any other class of vertebrate animals. The same 
bones as those found in other oviparous animals are almost always traceable. The gills are fur- 
nished with an Operculum or Gill-cover, on each side, the function of which is to close the 
aperture in case of need, and thus protect those delicate and important organs. There are several 
bones adapted to this particular organization. 
The teeth in fishes are entirely osseous, and are usually of a simple spine-like form. The 
