XX. MOVEMENT IN LOWER ANIMALS.' 



An examination of a muscle cell demonstrates that the 

 power of contraction and expansion which it possesses is 

 actually a property of the protoplasm of which 

 the cell is composed. We should expect, then, 

 to find in all living matter some power of 

 •movement, since all are built up of cells, that 

 is, protoplasm. In other words, motion is an 

 essential characteristic of all hving protoplasm. 

 It does not follow, however, that this mo- 

 tion of jDrotoplasm must necessarily result in 

 the movement of the animal whose cells con- 

 tain protoplasm, any more than the move- 

 ment of water in a bowl necessarily results 

 in the motion of the bowl. In fact, in many 

 of the cells of oiu- bodies and in those of 

 plant and animal bodies in general, the pro- 

 toplasm of the individual cells shows move- 

 ment, while the cells in which it is inclosed 



Pig 



126— A 



plant cell ; remain stationary. It is necessary, therefore, 

 indicare"the i^ Speaking of movement to distinguish be- 

 streaming of tweeu motlou of protoplasm by itself and 



protoplasm , . , 



within the locomotion, Or the movement of the body or 



'"'^- cell from one position to another. The motion 



of protoplasm within a stationary cell is very important 



as a means of distributing material within the ceU, but 



> See Footnote, p. 146, Chapter X. 

 294 



