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 motion 

 is observable in the higher animals. In our own Bodies 

 during the deepest sleep the breathing movements and the 

 beat of the heart continue; their cessation is to an onlooker 

 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 spontaneous motility. Many 

 a flower closes in the evening to expand again in the morning 

 sun; and in many plants comparatively 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 (Dioncea musci- 

 pula) 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 anirial, 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 ani- 

 mals can; but on the other hand some of the lower animals 

 are permanently fixed to one spot; and among the lowest 

 plants many are known which swim about actively through 

 the water in which they live. The lowest animals and plants 

 are in fact those which have undergone least differentiation 

 in their development, and which therefore resemble each 

 other in possessing, in a more or less manifest degree, all the 

 fundamental physiological properties of that simple mass of 



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