§ 3] GEOTAXIS 121 



To explain the phenomenon of axis-orientation, two principal 

 theories have been advanced. The first may be called the 

 mechanical theory; the second'ithe response-to-stimulus theory. 

 The first theory is that once '''suggested by Veewobk ('89, p. 

 122). It appeared to him that'^it was self-evident from purely 

 physical grounds that, in complete quiescence of the flagellum, 

 the hinder end of the protist should be directed downwards, 

 and not the anterior flagellum-bearing end. If one conceives 

 such an individual to move its flagellum, which precedes in 

 locomotion, it must move towards the surface of the water; 

 thus against gravity. Vbevp^Oen finds the stimulation theory 

 inconceivable, since gravity cannot even be compared with 

 stimuli. In falling, the body of the protist might rub against 

 the water particles, which would offer a stimulus, but this 

 would be more allied to rheotaxis. 



It might seem an easy thing to dfetermine whether geotactic 

 Protista artificially rendered quiescent (e.g. killed or stupefied) 

 would stand with their anterior ends uppermost; but the 

 killing is apt to distort the form, and the organisms being 

 heavier than water * fall to the bottom. Something might be 

 gained from an observation of how they fall, but there is very 

 great discordance among authors upon this point, probably in 

 part due to difficulties of observation. Thus Schwabz ('84, 

 p. 68) says that both Euglena and Chlamidomonas assume all 

 positions in falling; Massaet ('91, p. 164) finds that Chlamido- 

 monas falls with flagellum directed upwards and Jensen ('93, 

 p. 451) declares that Euglena viridis killed by iodine falls 



* i"ew determinations seem to have teen made of the specific gravity of living 

 Protista. Jensen ('93») attempted to do this for Parameoia, but his method was 

 bad and his results bad likewise. He made solutions of potassium carbonate, 

 of varying specific gravity, and found that Paramecium just floats in a solution 

 whose sp. gr. is 1.25. The difficulty of the method is that solutions of salt 

 having a relatively small molecular weight act so powerfully in withdrawing 

 water from the organism as to cause it to shrink and increase in relative 

 weight. Miss Platt has used solutions of gum arable whose osmotic action 

 is so slight that organisms live in it for hours. In such solutions, paralyzed 

 but living Spirostoma and Parameoia neither sank nor rose when the specific 

 gravity was between 1.016 and 1.019 ; so that it seems probable that the specific 

 gravity of Infusoria lies near 1.017. Tadpoles recently hatched and having a 

 length of 9.5 mm. had a sp. gr. of 1.044, while those 12 mm. long had a sp. gr. 

 of 1.017. 



