FISHES CONSIDERED AS A CLASS. 249 



which has at its origin a contractile bulb. Having been purified in 

 the gills, the blood is poured into an arterial trunk, situated under the 

 spine, whence it is distributed to every part of the body, and in due 

 time is returned by the veins to the auricle of the heart. Notwith- 

 standing that the circulation is single, that is, the blood passes through 

 the heart only once before passing through the body, it is complete, 

 since none of the blood circulates through the body till after it has 

 been purified in the gills. 



Respiration, as already indicated, is performed by means 

 of gills. These are delicate fringes or laminae, supported 

 on bony arcbes. In most species FIG. 239. 



the gills are covered by means of a 

 sort of lid composed of three pieces, 

 plainly or obscurely indicated, and 

 called the operculum, sitbojiieren- 

 lum, and interoperculum. This lid 

 or gill-cover plays on one called the 

 vreoverculum, In some groups, 



* p, preopercnhim ; o, opercu- 



however, the gill-covers are want- ] lim; s , eubopercuinm ; i, 



interoperculum. 



In the process of breathing, the mouth and gill-covers 

 open alternately, and the water which enters the month 

 passes through the gills and escapes at the gill-openings. 

 Thus the gills are constantly bathed with water, and the 

 oxygen needed is secured from the air which is mingled 

 with the water. 



Fishes, in most cases, are furnished with a membranous 

 sac filled with air, and called the swimming-bladder. This 

 organ is probably a rudimentary lung, but its true func- 

 tion is not known ; although some believe that by it Fishes 

 have the power of varying their specific gravity, and thus 

 more easily rising and sinking in the water. Others be- 

 lieve that it aids in hearing. 



The spinal column of Fishes is made up of vertebras 



11* 



