THE WHEEL ANIMALCULES (ROTATORIA) 585 



ing action of the pyroligneous acid, they may now be passed, like 

 other objects, through successively stronger grades of alcohol till 

 absolute alcohol is reached. They may then be cleared in clove or 

 cedar oil, in the usual way, and mounted in Canada balsam, or 

 gum damar, or they may be mounted in glycerine. 



In America the systematic work on the Rotifera has consisted 

 largely in the publication of lists of species found in certain regions. 

 While this work undoubtedly has its uses, there are other lines of 

 study which would at the present time be of much greater value 

 even for purely systematic purposes. On account of the very 

 large number of species of Rotifera, their minuteness, and the 

 unsatisfactory work that has been done upon them, it is often 

 almost impossible to determine with certainty even common spe- 

 cies. This can best be remedied by studying carefully circum- 

 scribed groups, such as single genera and families, collecting them 

 extensively, describing and figuring all the species, and going criti- 

 cally over the literature of the group in such a way as to set the 

 synonymy in order. 



Careful comparative studies of certain organs or sets of organs, 

 such as the corona, the trophi, etc., throughout varied groups, 

 would help much in understanding the interrelationships of the 

 Rotifera. If possible a study of the habits should be made in their 

 relation with the structures, since these matters are closely con- 

 nected. Monographic anatomical studies of certain species are 

 always of value. They would be especially useful if a thorough 

 study of the habit and physiology could be made at the same time. 



A most important field, and one little cultivated, lies in the 

 study of the activities by which the rotifers respond to their en- 

 vironment. Thorough studies of the movements and habits, the 

 reactions to stimuli, "tropisms," and the like, would be of great 

 interest. Disconnected observations on these matters are of com- 

 paratively little value; definite problems should be taken up and 

 followed to the end. 



The variations induced in a single species, and in an entire 

 fauna, by changes in seasons, temperatures, and in other features, 

 have received some study and deserve much more. One of the 

 most interesting lines of work for which the rotifers present oppor- 



