VERTEBRATA. 
319 
four limbs—the hinder the longeiyand the first developed. 
They have four fingers and five toes. The tongue is long, 
and, fixed by its an¬ 
terior end, it can be 
thrown out as 
an organ of prehen¬ 
sion. 163 The eggs are 
laid in the water en¬ 
veloped in a glairy 
mass; and the tadpoles FlG ' m °~ Frog ^ Rana) ' 
resemble the Urodelans, till both gills and tail are absorbed. 
Frogs (Band) have teeth in the upper jaw, and webbed 
feet; Toads (. Bufo) are higher in rank, and have neither 
teeth nor fully webbed feet. The former have been 
known to live sixteen years, and the latter thirty-six. 
Class III. —Reptilia. 
These air-breathing, cold-blooded Vertebrates are dis¬ 
tinguished from all Fishes and Amphibians by never haw 
ing gills, and from Birds by being covered with horny 
scales or bony plates. The skeleton is never cartilaginous; 
and the skull has one occipital condyle. The vertebrae are 
ordinarily concave in front; and the ribs are well devel¬ 
oped. With few exceptions, all are carnivorous; and teeth 
are always present, except in the Turtles, where a horny 
sheath covers the jaws. The teeth are never fastened in 
sockets, except in Crocodiles. The jaws are usually very 
wide. The heart has three chambers, save in Crocodiles, 
where the ventricle is partitioned. But in all cases a 
mixture of arterial and venous blood is circulated. The 
lungs are large, and coarsely cellular. The limbs, when 
present, are provided with three or more fingers as well 
as toes. 
There are about three thousand species of living Rep¬ 
tiles, and of these there are four main groups: the first 
rapidly 
