106 BIOLOGICAL LECTURES. 



rotricha : Stentor polymorphic, Spirostomum ambiguum, and 

 Bursaria truncatella ; of Hypotricha, Oxytricha fallax and a 

 number of undetermined species. In several of these creatures, 

 on account of the large size or other favorable circumstances, 

 it was possible to use methods of investigation not available for 

 Paramecium ; in particular it was possible in a number of cases 

 to localize very precisely the action of stimuli. 



In all of the organisms named, in spite of great variations in 

 the nature and complexity of the usual movements, the reac- 

 tion method was essentially similar to that of Paramecium. In 

 all, the direction of turning after a stimulus was toward a struc- 

 turally defined side, without regard to the nature and position 

 of the source of stimulus. With regard to the details of the 

 reaction, as might be expected, the greatest variety exists, but 

 the general reaction plan was the same throughout. 



This method of reaction evidently has a close relation to the 

 usual asymmetry of the cell body exhibited by these organisms. 

 This asymmetry of the Infusoria has also a close relation to the 

 normal method .of progression through the water, as well as to 

 the method of reaction to a stimulus. Most of these organisms, 

 as they swim forward, also revolve on the long axis, and the 

 resulting path is usually a spiral. The form of the body has a 

 constant relation to the axis of the spiral, the same side being 

 at all times directed toward this axis. The unsymmetrical 

 structure of the body, the usual method of progression, and the 

 method of reaction to a stimulus are thus evidently closely 

 interrelated. In the case of a bilaterally or radially symmetri- 

 cal animal one would certainly not expect that one side would 

 be always preferred to the other in turning away from a source 

 of stimulus, as is the case in the Infusoria. 



In the case of chemical stimuli it was found for all the organ- 

 isms studied that not only the turning to one side, but the 

 swimming backward after a stimulus, was independent of the 

 position of the source of diffusion of the chemical. The action 

 of chemical stimuli was localized by bringing a capillary glass 

 rod coated with some chemical compound near the anterior 

 end, one side, or the posterior end, of the quiet organisms. In 

 every case (except in Euglena viridis, which does not swim 



