MUSCULAR AND SKELETAL SYSTEMS. 



235 



limit to the size of a walking animal, and a much lower 

 limit to the size of a flying animal. Contrarily, as an 

 animal becomes smaller, the muscular energy per unit 

 section remaining the same, the weight decreasing 

 faster than the power, less and less proportion is neces- 

 sary for support of weight, and more and more is left 

 over for activity of all kinds. This is the true reason 

 why small animals seem so much more vivacious than 

 large animals. 



WORMS. 



In these there is no skeleton, but the muscles act 

 directly on the body to produce motion and locomotion. 

 We have, therefore, in these an entirely different mode 

 of action. 



Take an earthworm as a good example. In these 

 there are two kinds of muscular fibers, viz., the longitu- 

 dinal and the ring fibers. The longitudinal fibers, acting 

 all together, shorten the body ; acting on one side or an- 

 other, bend the body to the corresponding side. The 

 ring fibers constrict the body, and, acting all together, 

 elongate it. But the most conspicuous peculiarity of 

 all the fibers, both longitudinal and ring, is their propa- 

 gated action. Watch an earthworm in locomotion. We 

 observe a wave of constriction — i. e., contraction of ring 

 fibers — running forward, advancing each part consecu- 

 tively until it reaches the head, which then advances and 

 takes hold ; and then begins a wave of contraction of 

 longitudinal fibers, running backward and bringing up 

 successively the parts of the body toward the head. 

 Usually several such waves chase each other along the 

 body. This kind of motion is so characteristic of worms 

 that it may be called vermicular. It is found in all ani- 

 mals, even the highest vertebrates, in the involuntary 

 muscles, especially those of the stomach and intestines. 



