CHAPTER IX. 



THE STRUCTURE OF THE MOTOR ORGANS. 



Motion in Animals and Plants. If one were asked to 

 point out the most distinctive property of living animals,, the 

 answer would probably be, their power of executing spontane- 

 ous movements. Animals as we commonly know them are 

 rarely at rest, while trees and stones move only when acted 

 upon by external forces, which are in most cases readily re- 

 cognizable. Even at their quietest times some kind of mo- 

 tion is observable in the higher animals. In our own Bod- 

 ies during the deepest sleep the breathing movements and 

 the beat of the heart continue; their cessation is to an on- 

 looker the most obvious sign of death. Here, however, as 

 elsewhere in Biology, we find that precise boundaries do not 

 exist; at any rate so far as animals and plants are concerned 

 we cannot draw a hard and fast line between them with 

 reference to the presence or absence of apparently sponta- 

 neous motility. Many a flower closes in the evening to ex- 

 pand again in the morning sun; and in many plants compara- 

 tively rapid and extensive movements can be called forth by a 

 slight touch, which in itself is quite insufficient to produce 

 mechanically that amount of motion in the mass. The 

 Venus's flytrap (Dionaea muscipula) for example has fine 

 hairs on its leaves, and when these are touched by an insect 

 the leaf closes up so as to imprison the animal, which is 

 subsequently digested and absorbed by the leaf. The higher 

 plants it is true have not the power of locomotion, they 

 cannot change their place as the higher animals can; but 

 on the other hand some of the lower animals are perma- 

 nently fixed to one spot; and among the lowest plants many 

 are known which swim about actively through the water in 



