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Show 90 REPTILIA. SIREN, Lin. Sirens are elongated animals, almost anguilliform, with three bran· chial 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 adhering to two plates fixed under each side of the palate.(l) 8. lacertina, 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, occasionally going on shore or into the water. It feeds on lumbrici, insects, &c.(2) There are two much smaller species, 8. intermedia, LeConte, Ann. New York Lye., II, Dec. 1826, pl. 1. Blackish; four toes like the large one, but the branchial tufts are less fringed; its length does not exceed one foot. 8. striata, Le Conte, lb. I, pl. 4. Blackish; two longitudinal yellow streaks on each side; only three toes; the branchial tufts but slig·htly fringed; length nine inches.(3) (1) It is in vain that some authors have recently endeavoured to revive the an. cient idea, that the Siren is the tadpole of the Salamander. We possess speci· mens of them much larger than any known SalamandeJ•, whose bones have acquired their perfect hardness without the smallest vestige of hind feet; their osteology also differs widely from that of the Salamanders; they have more (90 ), and differently shaped vertebrz and fewer ribs (eight pairs); the conformation of the head, and the connexion of the bones which compose it, are altogether different. See Oss. foss. tome V, part II. (2) Barton denies that it feeds on Serpents, and that its voice resembles that of a young Duck, as affirmed by Garden. llarton, "S~me account of S. Lacert., &c." (3) The branchiz of these two species have been considered as taking no part in the process of respiration, in consequence of which M. Gray has formed a genus for them, which he calls PsEUDOBRANcaus; it is easy, however, on their inferior am·~·ace, to see folds and a vascular apparatus whose use is, to us, very plain; bestdes this, the observations of Major Le Conte demonstrate the fact that these S.irens, like the Lacertinidz, are perfect animals. ' 91 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 branchire, which consist of laminre suspended on arches that are attached to the hyoid bone, each composed of numerous separate laminre and covered with a tissue of innumerable bloodvessels. The water which the fish swallows, escapes between these laminre through the branchial openings, and by means of the air it contains, acts upon the blood that is continually arriving in the branchire 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, as that of the Bird for flight. Suspended in a liquid of nearly the same specific gravity as its own body, there was no necessity for large wings to support it. In a great number of species, immediately under the spine there is a bladder filled with air, which, by compression or dilatation, varies the specific gravity of the fish and assists it to rise or descend. Progression is effected by the motions of the tail, which, by striking the water alternately right and left. forces them forward; the branch ire, by impelling the water backwards, may also \ |