14 THE MECHANISM OF LIFE 



The Skeleton. The skeleton consists of a series of hard, rigi< 

 parts to which the muscles are attached in various ways; thiL 

 the chassis of a motor-car is, in a way, the skeleton of th 

 mechanism. The skeleton is also the mobile carrier of bodily 

 weapons, such as the teeth or claws, and a mechanical analogu 

 to it in this second aspect is the jib and scoop of a steam digging 

 machine, or the carrier of the buckets of a dredger. Not a' 

 animals possess skeletons, but in those which have soft bodie 

 there is always a rather small limit of size, and most of sue! 

 organisms live in water, which itself acts as the support for th 

 soft body parts. 



The animal skeleton varies to an enormous extent in its form 

 and sometimes it is very simple. Thus the skeleton of a musse 

 or oyster is a shell consisting of two valves connected together b] 



a kind of hinge, while tha 

 -Shell ligament of a man consists of a grea 



number of bones jointed o 



articulated together in man] 

 Adductor -,.. TT 



[muscle of the shell clitterent ways. Upon th 



nature of the parts of th 

 skeleton and the shapes of th 



"Shell joints or articulations depenc 



the kinds of movements tha 



Fio. l.-A TRANSYEESE SECTIOK may be carried out. Thus th 

 THROUGH AN OYSTER. skeleton of the oyster is ver 



simple, and so also is th 



nature of the only movement of the body, as a whole, whicl 

 this animal can make. One large muscle is attached to the tw 

 valves as indicated in the figure, and when this contracts i 

 pulls the valves together so that the shell remains closed 

 When it relaxes, the spring or elastic ligament presses th 

 valves apart, and so the shell opens. These opening an( 

 closing movements are practically the only ones that the oyste 

 can perform. Now compare with this simple mechanism th 

 very complex one of the vertebrate body as we are about t< 

 describe it. 



Sometimes the skeleton of an animal has no function in move 

 ment, but is merely the rigid, or semi-rigid, support of the sof 

 parts; thus the common bath sponge is the horny skeleton of ai 

 animal, and supports the fleshy tissues (which have been rottec 

 away in the process of preparing the sponge). This flesh i 



