

CHAP, iv REFLEX ACTION 39 



class, according to Dr. Verworn, make up a considerable 

 part of the life of Protista. 



" A Stentor draws itself suddenly together, stretches itself out, 

 draws in again after a short time, may remain stretched out a 

 long time without the slightest contraction, till presently it gives 

 vent to a whole series of contractions at irregular intervals." 



There is no appearance of purpose in these irregular 

 movements, and Dr. Verworn 1 points out that by im- 

 perceptible degrees they pass over into periodical, that is to 

 say, automatic movements. Automatic movement indeed, 

 as we see it in the pulsating vacuole among Protista, and 

 among higher animals in the beating of the heart or the 

 action of the respiratory centre, is a development and 

 adaptation of the persistent structural activity to special 

 requirements of the organism. It is due to the inter- 

 action of internal forces (though it may be modified by 

 outer influences) but it is repeated at regular intervals, or 

 rather, like the beating of the heart, it forms a connected 

 cycle of events each of which introduces the next, till at 

 length the cycle is completed and starts afresh. 



To understand movements of this class we must bear 

 in mind that in any living cell chemical changes are 

 constantly going on. The protoplasm of the cell is 

 building itself up out of materials supplied by the 

 surrounding medium. This is the assimilative process. 

 At the same time an opposite process of dissimilation 

 is going on, in which the constituents of the protoplasm 

 form new combinations, and ultimately become waste 

 products of which the cell gets rid. These changes 

 involving a constant molecular movement, the cell is 

 never in a condition of stable equilibrium. But the outer 

 results may be very different, according as the internal 

 changes do or do not tend to balance one another. 

 If they merely oscillate slightly about the point of 

 equilibrium, there will be no outward sign of change. 

 If there is a slow but decided gain of one set of forces 

 in one part of the cell, an amoeboid movement will 

 result. The cell will slowly put out a projection or 



1 Pro fist en Studicn^ p. 142. 



