20 PROTOZOA 



(i.) Mechanical stimuli. — Any sudden touch with another 

 body tends to arrest all motion ; and if the shock be protracted 

 or severe, the retraction of the pseudopodia follows. It is to 

 this reaction that we must ascribe the retracted condition of the 

 pseudopodia of most Ehizopods when first placed on the slide and 

 covered for microscopic examination. Free-swimming Protista 

 may, after hitting any body, either remain in contact with it, 

 or else, after a pause, reverse their movement, turn over and swim 

 directly away. This combination of movements is characteristic 

 as a reaction of what we may term " repellent " stimuli in 

 general.'- Another mechanical reaction is that to continuous 

 contact with a solid ; and the surface film of water, either at tlie 

 free surface or round an air-bubble, may play the part of a solid 

 in exciting it ; we term it " thigmotaxy " or " stereotaxy." When 

 positive it determines a movement on to the surface, or a gliding 

 movement along it, or merely the arrest of motion and prolongation 

 of contact ; when negative, a contact is followed by the retreat of 

 the being. Thus Paramecium (Fig. 55, p. 151) and many other 

 Ciliates are led to aggregate about solid particles or masses of 

 organic dihris in the water, which indeed serve to supply their 

 food. On contact, the cell ceases to move its cilia except those 

 of the oral groove ; as these lash backwards, they hold the front 

 end in close contact with the solid, at the same time provoking 

 a backward stream down the groove, which may bring in minute 

 particles from the mass. 



(ii.) Most living beings are able to maintain their level in 

 water by floating or crawling against G-RAVITY, and they react 

 in virtue of the same power against centrifugal force. This 

 mode of irritability is termed (negative) " geotaxy " or " barotaxy." 

 We can estimate the power of resisting such force by means of a 

 whirling machine, since when the acceleration is greater than 

 the resistance stimulated thereby in the beings, they are 

 passively sent to the sides of the vessel. The Flagellates, 

 Euglena and Chlamydoiaonas, begin to migrate towards the 

 centre when exposed to a centrifugal force about equal to -^ G 

 (G = 32'2 feet or 982 cm. per second) ; they remain at the centre 

 until the centrifugal force is increased to 8 G ; above that they 

 yield to the force, and are driven passively to the sides. The- 

 reaction ceases or is reversed at high temperatures. 



1 See Jennings in Woods Soil. Biol. Led. 1899, p. 93. 



