350 



ELEMENTARY BIOLOGY 



different, and each feather has a determinate growth ; that is, 

 there is a definite limit to the size and form which a single 

 feather can attain (Fig. 165). 



The bristles of hogs and the quills of hedgehogs and porcu- 

 pines are giant hairs. Hairs, quills, bristles, and feathers may 



FIG. 1 66. Box turtle 



The exoskeleton consists partly of skin plates and partly of bony expansion. This animal 



is protected not only by the withdrawal of head and limbs but by the further closing of 



the hinged breastplate, shown on the right 



be considered as special kinds of skin growths and may be 

 compared to the scales of the common fishes and of reptiles, 

 and to the plates found in the skin of -sturgeon and the gar 

 pike. The shield of the turtle or tortoise is in part a skin 

 structure and in part produced by the skeleton (Fig. 166). The 

 amphibians (frogs, newts, etc.) are the only backboned animals 

 that never produce outgrowths on the skin, although some of 

 the toads have irregular thickenings in the adult stage. 



