Page:Popular Science Monthly Volume 21.djvu/613

Rh endo- or vertebra-skeleton, is strangely modified and partly expanded upon the outside of the body; so it is literally true that the turtle is inside of his internal skeleton. The ribs are expanded to form the carapax, and the breast-bones to form the plastron of a solid box, within which many species can withdraw their head and limbs. But this is not all. The box is completed by the addition of numerous bony plates developed from the skin, and over all a horny epidermic



covering. The latter furnishes the beautiful and valuable tortoise-shell. Thus the armor of a turtle is a combination of 1, 3, and 4. One of the land-tortoises, the box-tortoise, deserves particular mention. The plastron, or breast-plate, of this species is divided into two movable parts hinged at a line drawn transversely or across the middle. When its head and feet are withdrawn, each end of the shell is tightly closed, so that no animal can get even a claw inside.

The skin of the rhinoceros is so very tough and thick that it defies ordinary weapons, and is said to resist soft-lead rifle-bullets.

Spines may be either (1) epidermic or (2) a secretion. The first class includes the spines of the echidna, hedgehog, and porcupine; also those of most fishes, lizards, and crabs. To the second class belong those of the globe-fishes, mollusks, rhizopods, and sea-urchins. The spines of the latter group have a remarkable structure. They are attached to the test or shell by a ball-and-socket joint and each moved by independent sets of muscles. While some species have only a very few large club-shaped spines, others have countless thousands of minute needles. The thorn-like spines of the common sea-urchin are also used in locomotion, and it has been happily said that a sea-urchin on its travels is like an animated chestnut-bur.

The hedgehog is one of the best protected of living animals. "Marching securely under the guardianship of its thorn-spiked armor, it recks little of any foe save man. . . . The formidable array of bristling spines with which the back is more or less covered offers a