154 SCIENCE FROM AN EASY CHAIR 



high-power microscope a little spherical liquid-holding 

 cavity, which slowly enlarges, then bursts at the surface 

 and collapses. After a brief interval it forms again, and 

 again bursts to the exterior. In the " bell-animalcule " 

 a beautiful active little creature only one-thousandth 

 of an inch in diameter it may be seen to form, swell, 

 collapse, and re-form as often as twenty times in a minute 

 (see Fig. 41). Soluble colouring matter taken in by the 

 animalcule with food is excreted by the liquid accumulated 

 in and ejected to the exterior by this spherical chamber. 

 It is called the " pulsating " or " contractile " vacuole, and 

 by its rhythmical pulsating movement of dilatation and 

 collapse presents definite points of similarity to the alter- 

 nately dilating and contracting hearts of higher animals. 

 The entering flow of liquid here, as in the veins and heart 

 of higher animals, is continuous. The rhythm is due, as 

 is the rhythm of the heart, to the alternation of a brief 

 period of activity or contraction, and a brief period of 

 consequent exhaustion, rest, and repair on the part of 

 living contractile substance. 



