MOTION AND LOCOMOTION 81 



the central nervous system, while ribs give pro- 

 tection to the heart, lungs and other important 

 organs. The exoskeleton of the turtle, armadillo, 

 &c., protects the entire body, the shell of the snail 

 and of the limpet and the integument of the beetle 

 perform similar functions, as do also the scaly hide 

 of reptiles, the hair of furry animals, &c. Simi- 

 larly, in plants, cork is protective, and even plants 

 which do not develop cork develop a thin corky 

 cuticle which is protective in function, while others 

 are provided with thorns, hairs, wax, resin, &c., 

 which are the analogues of an exoskeleton. 



Another function of the skeleton is to give rigidity 

 to soft parts which require it. Thus the thigh, leg, 

 arm and forearm bones give rigidity to these 

 members, while their jointing at the same time 

 permits freedom of movement ; the skeleton frame- 

 work of a leaf keeps its green substance expanded, 

 and so on. 



The skeleton of the animal, moreover, performs 

 a special function in that type of organism, in that 

 it gives points of attachment for muscles and enables 

 the individual parts to be moved independently or 

 collectively, and, at the same time, co-ordinately. 

 On the other hand, the skeleton of the plant may 

 perform a function which that of the animal does not 

 perform, namely, circulation, or the conveyance of 

 both crude and manufactured food materials from 

 one part of the organism to another. Manifestly, 

 it would be excessively inconvenient if the skeleton 

 of the animal acted also as a circulatory organ, for 

 circulation would be interrupted or impeded every 

 time the skeleton was put in motion ; in the plant, 

 on the other hand, economy of tissue is effected by 

 combining the function of circulation with that of 



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