444 



GENERAL PHYSIOLOGY 



stantly attracts new individuals chemotactically by its production 

 of carbonic acid ; thus all the individuals in the drop accumulate 

 finally about the foreign body (usually in the course of 5 -10 

 minutes), although, since it is surrounded by an impenetrable wall 

 of individuals held thigmotactically, most of them cannot come 

 into direct contact with it (Fig. 220, B). Thigmotaxis, which 

 causes the individuals that swim by chance to the foreign body to 

 remain, is merely the first cause of the assemblage ; chemotaxis 

 toward the carbonic acid produced by them completes it. 



A second form of baro taxis, in which the stimulus is produced, 

 not as in thigmotaxis by contact with a solid body, but by a gentle 

 current of slowly flowing water, is rheotaxis, which was discovered 

 by Schleicher and carefully investigated by Stahl ('84). This is 



FIG. 219. Oxytricha, a ciliate infusorian. A, Seen from below ; B, seen from the side ; C, creeping 

 about over the egg of a mussel. 



the peculiarity belonging to certain organisms, of taking toward 

 flowing water a direction of motion opposed to the direction of the 

 current. Since these organisms thus turn toward a pressure- 

 stimulus, rheotaxis is merely a special form of positive barotaxis. 

 Thus far rheotaxis is known in a few organisms only. Stahl 

 demonstrated it best in the plasmodia of Myxomycetez in Aethalium 

 septicum, by the following experiment. He suspended a narrow 

 .strip of filter-paper in a beaker filled with water, and somewhat 

 elevated, in such a manner that one end of the strip dipped into 

 the water, while the other end hung far down over the edge of the 

 beaker. In such a strip there is a continuous slow current of 

 water directed toward the end that hangs down, as is proved by 

 placing upon it a coloured mark. Stahl laid this end upon a mass 

 -of tan, in which the plasmodia of Aethalium live. The result was 



