14 



THE MECHANISM OF LIFE 



The Skeleton. — The skeleton consists of a series of hard, rigid 

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

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

 mechanism. The skeleton is also the mobile carrier of bodily 

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

 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 all 

 animals possess skeletons, but in those which have soft bodies 

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

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

 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 mussel 

 or oyster is a shell consisting of two valves connected together by 



a kind of hinge, while that 



-Shell ligament ^^ ^ ^^^ consists of _ a great 



number of bones jointed or 

 articulated together in many 

 different ways. Upon the 

 nature of the parts of the 

 skeleton and the shapes of the 

 joints or articulations depend 

 the kinds of movements that 

 may be carried out. Thus the 

 skeleton of the oyster is very 

 simple, and so also is the 

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

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

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

 pulls the valves together so that the shell remains closed. 

 When it relaxes, the spring or elastic ligament presses the 

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

 closing movements are practically the only ones that the oyster 

 can perform. Now compare with this simple mechanism the 

 very complex one of the vertebrate body as we are about to 

 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 soft 

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

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

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



"lAdducfor 

 'muscle oFfhe shell 



Shell 



Transverse Section 



THROUGH AN OySTER, 



