CLASS IV. FISHES. 65 



are strictly an aquatic animal, and capable of breathing and 

 living only under water. They are furnished with gills like 

 a fish, and have no legs, but are provided with a tail, which 

 serves them as an instrument of locomotion. In this state 

 they are seen by thousands, of a dark color, with round 

 bodies, swimming about in brooks, and small ponds ; and are 

 known by the familiar name of Tadpoles. After a certain 

 period, their form and structure are altered ; their fefet and 

 legs grow, and project from beneath the skin ; their tail, their 

 gills, and the covering of their head, fall off; they begin to 

 respire by means of lungs; and become, at length, animals 

 capable of breathing only in the air. This transformation is 

 not, however, in all cases complete. In two genera, the Pro- 

 teus and the Siren, besides lungs, the gills are retained 

 through life, and they are thus possessed of two distinct sets 

 of organs of respiration. 



SECTION V. 

 Class IV. Fishes. 



FISHES, being destined to inhabit only the water, are pro- 

 vided with organs and a structure adapted to the element in 

 which they reside ; arid, since they cannot breathe air, of 

 course some modification in the organs of respiration and cir- 

 culation is required to enable them to perform those functions. 

 The heart, in them, has only one auricle and one ventricle. 

 The blood, coming from the body, is received into the auri- 

 cle, and transmitted by means of the ventricle to the gills, 

 which perform the same office as lungs. These are situated 

 upon each side of the neck, and consist of semicircular arches 

 of bone or cartilage, to which are attached membranes, divided 

 into little fibrils or fringes, to which the blood is distributed, in 

 very small vessels, after it comes from the heart. Over the 

 gills a constant current of water is passed, by the action of the 

 mouth of the animal, which, by means of the air thai it con- 

 tains, exerts an influence over the blood circulating in them, 

 and produces the same changes in it as are produced in the 

 lungs of other animals by the air they breathe. From the 

 gills, the blood does not return to the heart, but is collected 

 into one large artery, which passes down along the spine, and 

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