THE SKIN AND SKELETON 



343 



the parts together. They can not grow together, like a 

 broken bone. 



Embedded in the back of the cuttlefish is a very light 

 spongy "bone," which, as already observed, is a secre- 

 tion from the skin, and therefere belongs to the exo- 

 skeleton. It has no resemblance to true bone, but is 

 formed, like shells, of a number of calcareous plates. 

 Nevertheless, the cuttlefish does exhibit traces of an 

 endoskeleton : these are plates of cartilage, one of which 

 surrounds the brain, and hence may be called a skull. 

 To this cartilage, not to the "cuttlebone," the muscles 

 are attached. 



In vertebrates, the exoskeleton is subordinate to the 

 endoskeleton, and is feebly developed in comparison. 

 It is represented by 

 a great variety of ap- 

 pendages to the skin, 

 which are mainly or- 

 gans for protection, 

 not for support. 

 Some are horny 

 outgrowths of the epi- 

 dermis, such as hairs, 

 feathers, nails, claws, 

 hoofs, horns, and the 

 scales of reptiles ; 



others arise from the hardening of the dermis by cal- 

 careous matter, as the scales of fishes, the bony plates 

 of crocodiles and turtles, and the shield of the armadillo. 



The scales of fishes (and likewise the spines of their 

 vertical fins) lie embedded in the overlapping folds of 

 the skin, and are covered with a thin, slimy epidermis. 

 The scales of the bony fishes (perch, salmon, etc.) con- 

 sist of two layers, slightly calcareous, and marked by 

 concentric and radiating lines. Those of the shark 



FIG. 298. Skeletal Architecture in the Armadillo, 

 showing the relation of the carapax to the verte- 

 bral column. 



