PROTECTIVE ARMORS 



347 



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

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

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

 of plates, and in projecting spines (see Fig. 161). 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). The 

 bark cells produced on the 

 outside of this cambium 

 layer soon die, and the 

 walls become corky. As 

 new layers are produced 

 underneath, the old layers 

 are moved farther and 

 farther from the center 

 of the plant. On the out- 

 side the dead cells, 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 



FIG. 162. Hairs of plants 



<7, branching hair on leaf of tobacco plant ; t>, hair 

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

 stalk of Chinese primrose ; d, marginal tooth on 

 sedge leaf ; e, glandular hair on flower of hop ; 

 /, leaf of apple of Sodom ; g, stinging nettle, 

 with tip, greatly enlarged 



