CHAPTER VI 

 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 spontaneous 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 recognizable. 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 ab- 

 sence 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 fly-trap (Dioncea 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 

 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 protoplasm which formed the starting-point of 

 each individual. With the physiological division of labor which 



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