VERTEBRATA. 811 
all the viscera, but also the whole muscular system, which 
is attached internally, as in Articulates; and even the 
limbs are inside, instead of outside, the thorax. The exo- 
skeleton unites with the endoskeleton, forming the cara- 
pax, or ease, in which the body is inclosed. The exoskele- 
ton consists of horny plates, known as “ tortoise-shell ;” in 
the soft Tortoises 
(Trionyx) this is 
wanting. The ver- 
tebrze of the back 
are soldered togeth- 
er, and the ribs are 
expanded, making 
the walls of the we 099,-Box-tortoise (Cistudo steps ne 
carapax. The ven- States. 
tral piece is called the plastron, or sternum.” All are 
toothless. There are always four stout legs; and the or- 
der furnishes the only examples of Vertebrates lower than 
Birds that really walk, for Lizards and Crocodiles wriggle 
and drag their body along. The eggs are covered with a 
calcareous shell. 
The Sea-turtles, as the edible Green Turtle and the 
Hawk’s-bill Turtle, which furnishes the “ tortoise-shell” of 
commerce, have the limbs converted into paddles. The 
fresh - water forms, represented by the Snapping Turtle 
(Chelydra), are amphibious, and have palmated feet. 
Land Tortoises (Zestwdo) have short, clumsy limbs, fitted 
for slow motion on the land; the plastron is very broad; 
and the carapax is arched (while it is flattened in the 
aquatic species), and head, legs, and tail can be drawn 
within it. The land and marine species are vegetable- 
feeders; the others, carnivorous. 
4. Crocodiles, the highest and largest of Reptiles, have 
two exoskeletons—one of horny scales (epidermic), and 
another of bony plates (dermal). The bones of the skull 
