378 FISHES. 



The greater part of Fishes, besides having their bodies ad- 

 mirably constructed for the liquid in which they move, have 

 in addition a singular apparatus, for rendering them specifically 

 lighter or heavier than water. This consists in a membranous 

 sac containing air, called the air-vessel or swimming-bladder, 

 which is placed under the spine, and by compressing or dilating 

 which they are supposed to rise or sink in the water. This ves- 

 sel forms two compartments in the carp, and often communicates 

 with the intestines in fishes which inhabit lakes or rivers. 



One of the chief differences in structure which characterizes 

 the present class of animals is their respiratory apparatus. Liv- 

 ing in a fluid element, their respiration is necessarily adapted 

 to the nature of that fluid. This is accomplished by means of 

 an apparatus named gills or branchiae, placed on the sides of 

 the neck or head. These branchiae consist of numerous lami- 

 nae, suspended on arches attached to the hyoid bone, each com- 

 posed of a great number of separate laminae, covered with a 

 tissue of innumerable blood-vessels. The water which they 

 swallow passes between these laminae, and escapes by the bran- 

 chial openings. In its passage, the blood which is sent from 

 the heart to the gills is acted upon by the air contained in the 

 water. After undergoing this change, it is collected in an arte- 

 rial trunk situate under the spine, which, though resembling in 

 anatomical situation the aorta of animals with double heart, per- 

 forms the functions of a left ventricle, and distributes it by nu- 

 merous ramifications through the body, from which it returns 

 to the heart by the veins. The branchial openings are cover- 

 ed either by an osseous moveable plate, which is termed the gill- 

 cover or operculum, or by a simple membrane, with one or more 

 openings. 



The whole of the blood in fishes is sent by the heart to the 

 branchial vessels, and is then venous or dark blood ; but when 

 it has been exposed to the air in the water, it assumes the ar- 

 terial or red colour, and passes into other vessels, which unite 

 in the great arterial trunk under the spine. Thus the heart 

 has but a single auricle, a single ventricle, and a single artery ; 

 and it is believed, that the little portion of heat developed in 

 this mode of respiration is owing to the small quantity of air 

 to which the blood is at one time exposed in passing through 

 the branchiae. 



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