THE MANIFESTATIONS OF LIFE 



73 



the contained nutritious matter, and of enabling the 



nutritious matter to be acted 



upon and transformed 



by the enzymic substances 



contained in the cytoplasm. 



It is probably indispensable 



to the phenomena of molec- 



ular exchange constituting 



metabolism, and may there- 



fore be presumed to be in 



uninterrupted progress in 



every active living cell. The 



more active the cell, the 



greater the need of such cir- 



culatory movements and the 



more active they become. 



In the higher protozoa the 

 cytoplasmic circulation is 

 facilitated by certain organs 

 known as contractile vacu- 

 oles. Thus if one of the 

 large vacuoles in a Para- 

 mcecium be carefully watch- 

 ed it will be found to undergo 

 a rhythmical change of place, 

 becoming rapidly smaller 

 and disappearing where first 

 seen, to appear and grow 

 correspondingly larger at a 

 new situation. After a given 

 interval contraction again 

 takes place and the vacuole 

 reappears in the original situ- 

 ation as it disappears from 



. , , Fm FIG. 18. Two cells and a part of a 



its recent one. These vacu- third from the tip of a .. leaf .. of a 



thus perform a func- stonewort, showing rotation of the 

 Suggestive Of a primi- Protoplasm in the direction of the 

 , . r arrows. (Sedgiwck and Wilson.) 



tive heart, keeping the cyto- 



plasm constantly agitated by currents of circulating fluid. 



