CLASS PISCES. Si 



site side of the head (b'). The heart (h) has the function of 

 driving the impure blood to the gills, and it consists of 

 two chambers or cavities only. 



The process of respiration or breathing is carried on by 

 the Perch as follows : The gills, as we have just described 

 them, are placed on the sides of the throat ; but, instead 

 of being exposed to view, they are concealed and protected 

 on each side by a kind of movable door or flap, composed 

 of a series of flattened bones, and known as the " gill- 

 cover" (fig. 33, o). They are also protected below by a 

 membrane, which is supported by slender, curved, bony 

 spines. In this way the gills occupy a sort of chamber 

 on each side of the neck. Internally, each of these 

 chambers opens into the throat by a series of slits, already 

 alluded to ; and externally, each opens upon the surface 

 by a vertical slit or fissure, which is placed just behind 

 the gill-cover, upon the side of the neck, and which is 

 known as the gill-slit. When the Perch wishes to breathe, 

 which it does several times in the minute, it opens its 

 mouth and takes in a gulp of water ; it then forces the 

 water into the gill-chambers through the slits in the 

 throat ; and finally, after the gills have extracted all the 

 oxygen from it, the fish opens the gill-cover and expels 

 the now airless water through the gill-slit. 



The Perch also possesses a singular membranous sac or 

 bladder, which is termed the " swim-bladder," and is placed 

 below the spine, behind the head. This is filled with 

 gas, and appears to have the function of enabling the 

 animal to rise and sink in the water at will. It is to be 

 regarded, however, as really corresponding with the limgs 

 of the air-breathing vertebrate animals. 



The nervous system of the Perch consists mainly of the 

 spinal marrow, protected by the backbone, and the brain, 

 contained within the skuU. As organs of sense, the ani- 

 mal possesses two eyes, which are destitute of eyelids ; 

 ears which are placed internally, and have no outward 

 opening ; and a nose, opening by nostrils on the top of 

 the snout, but having the peculiarity that it does not 

 open into the throat behind. 



