ANIMAL SELF-DEFENSE. 



597 



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 



Fig. 4. Armadillo (ChlamyphorKS truncatus). 



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 mov- 

 able 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 When its head and feet are withdrawn, each end of the shell is tightly 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 touo-h 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 vany foe save man. . . . The formidable array of 

 bristling spines with which the back is more or less covered offers a 



