The Tortoise--so called from its twisted feet, or its crooked way of walking--has, as you know, an upper and an under shell which covers its body like a coat of mail, protecting it from every enemy except man. This strong shell is, like that of the snail, a house for the tortoise to live in; but this house is formed by arched bones, and is part of the creature itself. The four feet of the tortoise or turtle, and a curious mouth rather like the beak of a bird--without teeth, but with jaws hard enough to make a bite from it very painful--and a little scaly tail; these are the only parts of the animal not covered by the shields of its bony shells. The Lizard has both limbs and teeth, but no shell. Lizards are wonderfully active, darting away at the least alarm, so that it is not easy to catch them. We may think of the Crocodile of the African, and the Alligator of the North American rivers, as enormous lizards; though they are now placed in a class by themselves, on account of their horny covering, which is so strong that it is almost impossible to pierce through it, and so smooth that a bullet will glance off from it. Serpents have neither shell nor limbs. Their vertebræ, as you will see, if you look at any skeleton of a snake in the Museum, fit very beautifully one into the other; and owing to this they are able to glide swiftly along the ground, to coil their shining length round trees, and to dart their heads in every direction. In one respect Tortoises, Lizards, and Serpents are alike--they all lay eggs, only the shell is not made of lime and earth, but is soft like leather. They are also cold-blooded animals, like fish. Of tortoises, some live on land, some in marshy places, some in rivers; turtles live in the sea, their lungs being so made as to enable them to remain under water without breathing.