Let us make this clear, for it is a strange history. If you look at the skeleton of a lizard (Fig. 23, p. 103), it is all straight-forward enough. His head fits on to his long-jointed backbone, which is able to bend in all parts freely, down to the very tip of his tail. His front legs with their shoulder bones (_s_), and his hind legs with their hip bones (_h_), are attached in their proper places to his backbone, and lastly, his ribs (_r_) protect the inside of his body, and by expanding and contracting pump the air in and out of his lungs, the front ribs being joined underneath in a breastbone. It is easy to see, therefore, that the lizard may be active and nimble, twisting his body hither and thither, and escaping his enemies by his quickness. But the tortoise is slow and sluggish, and has only managed to baffle the numberless animals which are looking out for a meal by fabricating a strong box to live in. But he had to make this out of the same kind of skeleton as the lizard, with the one difference that he has no breastbone. Let us see how it has been brought about. The bones of his neck are jointed and free enough as you can see (Fig. 21), and so are the joints of his tail, beginning from behind his hip bones (_h_). But with his back it is different. The backbone can be clearly seen inside the empty shell, running from head to tail so as to cover the nerve-telegraph, but the joints (_j_) have all grown together, and on the top they have become flattened into hard plates,[74] while the ribs (_r_) which are joined to them have also been flattened out and have grown firmly together so as to make an arched cover or _carapace_. If now you look at the back of the young tortoise (Fig. 22), which has been taken out of the egg before it was full-grown, you will see these plates (_p_) on the side where the tortoise-shell (_ts_) has been peeled off. They have not yet widened out enough to be joined together, and the ribs (_r_) are as yet only united by strong gristle. But what is that row of oblong plates (_mp_) round the edge? Those are the marginal plates, and they are mere skin bones, like the bony plates of the crocodile, but they are all firmly fixed together so as to bind the edges of the ribs, while plates of the same kind form the shell under the body, and the whole is covered by the horny skin. [Illustration: Fig. 22. Back of a Young Tortoise.--(_From Rathke._) _ts_, Tortoise-shell covering the whole carapace; this has been removed on the right side; _mp_, marginal plates binding the edges of the ribs; _np_, neck-plate; _p_, plates formed of the top of the backbone joints which have grown together; _r_, ribs which have not yet spread out so as to form a continuous shell; _lm_, _lm′_, front and hind leg muscles not yet covered by the carapace. ]