Effects of Desiccation on the Rotifer 221 



of these states in a given individual have not yet been worked out 

 in detail although the problem presents a number of points of 

 interest. A swimming individual behaves in much the same man- 

 ner as the forms described by Jennings ('04) ; orientation in response 

 to any stimulus takes place by random movements resembling 

 those of an infusorian. In a creeping individual, on the other 

 hand, orientation is much more direct; the process resembling 

 that which occurs in a planarian. Any strong sitmulus causes 

 the animal to stop and make a few testing movements with its 

 proboscis; orientation either towards or away from the stimulus 

 then occurs directly without the intervention of random move- 

 ments. If the stimulus be very strong, the animal contracts at 

 once, this reaction being caused by a variety of stimuli such as 

 heat, injurious chemicals, hypertonic solutions, mechanical shocks 

 and the onset of desiccation. It is interesting to note that in this 

 one animal we have at one time the method of reaction of an infu- 

 sorian and at another that of a typical metazoan. As has already 

 been mentioned, the temperature and the purity of the water seem 

 to be two of the factors which determine whether creeping or swim- 

 ming shall prevail, although probably they are not the only ones 

 concerned. Feeding movements may occur under almost any 

 conditions; they are especially marked after a period of desiccation. 

 The reaction of Philodina to light is of some interest. Normally 

 it is almost indifferent. When a number of individuals are pres- 

 ent in a dish they tend to become scattered to all parts of it regard- 

 less of the direction of the source of light. If now they be dis- 

 turbed, either by jarring the dish, drawing the water through a 

 pipette a few times, or adding a few drops of fresh water, they 

 immediately move towards the side of the dish away from the light, 

 either creeping or swimming according to circumstances. This 

 reaction is very striking and never fails to occur. In the course 

 of these experiments it was put to a practical use whenever it was 

 desired to obtain rotifers free from sand. A small amount of the 

 sand from the large culture was placed in a small dish and the 

 water agitated with a pipette, being given a spiral motion so as to 

 carry the sand to the center of the dish. In a few moments most 

 of the rotifers had invariably collected in the clean water on the 



