346 



ELEMENTARY BIOLOGY 



In many plants the outer surface of the leaf or fruit has, 

 in addition to the cutin, a layer of waxy material. This is 

 the bloom that we see on plums and other plant surfaces. 



In many animals the cells forming the surface layer of 

 the body are small and thick-walled, and many kinds of 

 secretions add to the protective value. The horseshoe crab 

 (Fig. i6o) produces his armor by secreting a substance that 



hardens like a varnish in the 

 water. This is very similar to 

 the substance that makes up 

 the exo-skeleton (outside skele- 

 ton) of insects, the chitin (pro- 

 nounced ki'tin), which is also 

 formed by the secretion of the 

 skin cells. 



In lobsters, crabs, crayfish, 

 and their relatives (the Crusta- 

 cea) the chitin secretion is com- 

 bined with a comparatively large 

 amount of carbonate of lime. 

 This it is that gives the exo- 

 skeleton of these animals their 

 crusty quality. Clams, oysters, and snails have extremely soft 

 skins. (The name of this whole group of animals, Molhisca, 

 refers to the general softness of these organisms.) They receive, 

 however, a great deal of mechanical protection from their shells, 

 which consist of deposits of lime formed by the secretions of a 

 special fold of tissue called the mantle (see Fig, 44). 



On the clam and on the snail the lines indicate the successive 

 deposits of lime. The inner surface of the shell is often very beauti- 

 ful and iridescent because of the very fine lines that break up the 

 surface. This mother-of-pearl is used extensively for ornamental 

 purposes. — for buttons, knife-handles, etc., — and the shells of many 

 mollusks are used for their hardness and durability in the making of 

 buttons and similar objects, without regard to their beauty. 



Fig. 161. Sea urchin 



Animals of this branch deposit large 



quantities of lime in their skin, and 



produce knobs and spines that form a 



protective armor 



