PROTECTIVE ARMORS 



347 



In the starfish and sea urchins and their relatives {Echino- 

 dermata, meaning "spiny-skinned") the slvin secretes a great 

 deal of lime, which is deposited in the form of definite rows 

 of plates and in projecting spines (see Fig. i6i). We may 

 well imagine that no fish 

 would care to eat a mouth- 

 ful of such spiny creatures 

 as the sea urchin or to bite 

 and swallow the harsh rays 

 of a starfish. 



In the trunks of our 

 common trees there is a 

 growing layer that con- 

 stantly produces new layers 

 of wood and new layers of 

 bark (see Fig. 63), which 

 consists at first of epider- 

 mal cells, pith cells, and 

 phloem. As the plant con- 

 tinues to grow, the older 

 cells die and their walls 

 become corky. The older 



Fir,. 162. Hairs of plants 



layers are moved farther i?, branching hair on leaf of tobacco plant; /», hair 



, on leaf of thorn apple ; c, glandular hair on leaf- 



and farther from the center g^gij^ ^f Chinese primrose ; d, marginal tooth on 



of the plant as new layers sedge leaf ; c, glandular hair on flower of hop ; 



11/1 ^^^^ °f apple of Sodom: g, stinging nettle, 

 of cells are formed, and with tip, greatly enlarged 



the outermost layers, ex- 

 posed to the weather and to mechanical injury from moving 

 animals and other objects, rub off or chip off. The mass of 

 bark is thus a constantly renewed protective layer. 



Similar in some ways to the bark of a tree is the hide 

 or skin of a mammal. Our own skin, for example, is made 

 up of dead cells on the outside. These are constantly rub- 

 bing off, but are as constantly replaced by new cells from 

 beneath. The growing layer (see b. Fig. 92) gives rise 'to 



