BATRACHIA. 197 



Laur. More than a foot long, about the thickness of a finger, with a verti- 

 cally compressed tail and four small legs. 



Finally, there are some which are possessed of fore feet only, the hind 

 ones being entirely deficient. They form the genus 



SIREN, Lin. 



Sirens are elongated animals, almost anguilliform, with three branchial tufts; 

 they have no hind feet, nor is there even a vestige of a pelvis. Their head 

 is depressed, the opening of their mouth small, their muzzle obtuse, eye 

 very small and ear concealed; the lower jaw is armed with teeth all round, 

 and there are none in the upper one, but there are several rows of them ad- 

 hering to two plates fixed under each side of the palate. 



S. locertina, L. Blackish, and attains the length of three feet; four toes 

 to each foot; tail compressed into an obtuse fin. It inhabits the marshes of 

 Carolina, the rice swamps particularly, where it lives in the mud, occasion- 

 ally going on shore or into the water. It feeds on lumbrici, insects, &c. 

 There are two much smaller species. 



CLASS IV. 

 PISCES. 



The class of Fishes is composed of oviparous Vertebrata with a 

 double circulation, but in which respiration is altogether effected 

 through the medium of water. For this purpose, on each side of 

 the neck, they have an apparatus called branchiae, or gills, which 

 consist of laminae suspended on arches that are attached to the hyoid 

 bone, each composed of numerous separate laminae and covered 

 with a tissue of innumerable blood-vessels. The water which the 

 fish swallows, escapes between these laminae through the branchial 

 openings, and by means of the air it contains, acts upon the blood 

 that is continually arriving in the branchiae from the heart, which only 

 represents the right auricle and ventricle of warm-blooded animals. 

 This blood, having received the benefit of respiration, is poured into 

 an arterial trunk situated under the spine, which, exercising the 

 functions of a left ventricle, distributes it to every part of the body, 

 whence it returns to the heart by the veins. 



The entire structure of the Fish is as evidently adapted for natation, 



