348 LOUIS MAX HICKERNELL. 



be the purity of the medium in which it lives, the amount of 

 food material present and the temperature of the medium. If 

 the water surrounding the animals does not contain too much 

 organic material, they usually may be seen stretched out, fastened 

 by the foot and going through the usual feeding movements. 

 When conditions are unfavorable they contract (PI. II., Fig. 4) 

 and remain quiet until the surroundings become more favorable. 

 A slight increase in temperature is usually conducive to more 

 vigorous movements, especially swimming. 



In preparing P. roseola for microscopic examination or subse- 

 quent sectioning some difficulty may be experienced because of 

 the fact that the animals are generally associated with sand or 

 other foreign material. When the animals are present in suf- 

 ficient numbers the method of collection suggested by Jacobs is 

 perhaps the best. This takes advantage of the reaction of the 

 animals to light. If subjected to no mechanical stimulus they 

 usually are indifferent to natural light of ordinary intensity. 

 However, if the culture is violently agitated by shaking or 

 stirring they become temporarily negatively phototropic and 

 may be collected with a pipette from the darker part of the 

 culture. When it was desirable to select individuals of any 

 given size or condition of maturity in these experiments, or to 

 get some absolutely free from foreign material this was accom- 

 plished by picking up the animals one by one from a Syracuse 

 watch glass with a capillary pipette, the entire process being 

 watched with a binocular microscope. The foregoing statement 

 is not to be interpreted as meaning that P. roseola is ever covered 

 with debris. Such is not the case. The foreign material alluded 

 to consists of bits of sand, etc., to which the animals adhere by 

 means of the sticky secretion of the foot glands. 



When first put on the slide under a cover glass the animals are 

 generally so active that a single individual can be kept within 

 the field of a high power lens only with greatest difficulty. As 

 some of the water evaporates from under the cover glass, and as 

 the latter gently presses upon the animal, movements become 

 less rapid and many details of organization can be made out 

 with little trouble. 



By putting the rotifer in a weak solution of neutral red some 

 of the internal organs are stained but not with any particular 



