REACTIONS TO STIMULI IN CERTAIN ROTIFERA. 



79 



cellus Gosse, D. gracilis Tessin, and a number of other species of 

 Rattulidae ; Plcesoma Icnticulare Herrick, Cathypna ungulata Gosse, 

 Monostyla bulla Gosse, Brachionus pala Ehr., Anurcea cochlearis 

 Gosse, and A. aculeata Ehr. 



This method by no means exhausts the possibilities of reaction even 

 to a simple mechanical stimulus in these species. They may retract 

 the head and cease swimming, may creep over the surface of the object 

 with which they come in contact, or possibly may sometimes turn other- 

 wise than to the dorsal side when stimulated. Of this latter point I 

 am, however, by no means sure. It is certain that the typical reaction, 

 occurring in the great majority of cases, is that described above. 



REACTION TO CHEMICALS. 



The reaction given when the organism comes in contact with an area 

 containing a rather strong 

 diffusing chemical was 

 observed in Aletopidia 

 lepadella, Anurcea coch- 

 learis, A. aculeata, and 

 Diurclla gracilis. 



The method of experi- 

 mentation was as follows : 

 A drop of water contain- 

 ing the rotifers was placed 

 on a slide. Near this was 

 placed a drop of N/S 

 NaCl, and the two drops 

 were connected by a nar- 

 row neck. The behavior 

 of the organisms as they 

 came into the region of the neck and thus in contact with the salt 

 solution was observed with the Braus-Driiner microscope. In the 

 species mentioned the reaction was by a sudden turn toward the dorsal 

 side, by which the path of the animal was directed away from the 

 chemical. The reaction is thus of the same character as occurs in the 

 ciliate infusoria. 



This manner of reaction to chemicals is in both these groups of 

 organisms just what might be expected when the currents caused by 

 the cilia are taken into consideration. In the ciliate, as I have shown 



FIG. 27.* 



*FiG. 27. Diagram of currents in a nearly quiet Anursea, showing how a 

 diffusing chemical or an advancing region of warmer water (represented by shad- 

 ing), is drawn out by the ciliary vortex, so as to reach the mouth and the ventral 

 surface before affecting other parts of body. 



