ICHTHYOLOGY. 545 



other like the teeth of a comb, but sometimes arranged in bunches. 

 These organs are analogous to lungs in terrestrial animals, being 

 adapted to extract from the air contained in the water, the oxy- 

 gen which is needed for the renewal of the blood. The breath- 

 ing apparatus formed by the gills is double in form, placed on 

 each side of the neck. Most commonly it consists of several 

 series of laminae, or membranous plates, fixed upon slender 

 arches of bone. Over these thin membranous plates branch out 

 innumerable blood vessels, whose walls are so thin as to permit 

 the fluid contained in them to absorb the oxygen with which they 

 are brought in contact when the fish takes in water through the 

 mouth. In order to carry off the water when deprived of its oxy- 

 gen, and to bring in fresh portions to be successively respired, 

 a constant current is produced over the surface of the gills, by 

 the action of the fish while taking in water at the mouth, and 

 throwing it out on each side, behind the gills, through orifices which 

 it has for the purpose, called the gill-openings. The apparatus for 

 breathing is protected by large bony plates, or opercular bones, 

 making up the chief portion of the sides of the head. These are 

 four in number, and are termed the operculum, the siib-oper- 

 culum, the pre-operculum, and the inter-operculum. The first 

 of these covers the gills. The Iranchiostegous rays, often 

 mentioned in descriptions, are situated under the opercular 

 bones. In the Sharks, Squalida, (Lat. squalus, a sea-fish,) and 

 the Rays, Raida, (Lat. raid, a ray,) the gills are attached at 

 their outer margin, with a separate orifice to each, through which 

 the water escapes. The orifices, usually five in number, are, 

 with the mouth and nostrils, on the under surface, and completely 

 hid when the fish is laid on its belly. 



The heart consists of but one auricle and one ventricle. The 

 blood collected from the venous system, is accumulated in the 

 single auricle, thence it is sent into the ventricle, and this drives 

 it into the gills where it is changed from venous to arterial blood, 

 and thence circulated through the body in arteries, aided by the 

 contraction of the surrounding muscular fibres. Hence it will 

 be perceived the heart never contains any but venous blood, the 

 arterial first proceeding from the gills. 



Most of the bony fishes have a membranous bladder, com- 

 monly of a lengthened form, placed along the body between the 

 spine and the bowels, and having a structural relation to the 

 lungs in the higher Vertebrates. This is filled with air, and 

 well known as the air-bladder, or swimming-bladder. When- 

 ever possessed, it aids more or less the process of respiration. It 

 also serves another important purpose, which is to enable the fish 



