332 FRESH-WATER BIOLOGY 



slides are convement for work with large forms. Anesthesia may 

 be induced by the use of a solution of one-tenth of one per cent 

 of chloretone, or even less with some species. For preservation 

 hot corrosive subhmate may be used, or a cold solution of the 

 subHmate to which five per cent of glacial acetic acid has been 

 added. Lang's fluid, Chichkoff' s mixture, and 30% HNO3 fol- 

 lowed after one minute with 70% alcohol, are all useful kilhng 

 reagents. Formol is useful for preservation of external characters 

 since the animals retain their shape and color in it better than 

 in most reagents. The larger planarians are especially valuable 

 for study in laboratories where attention is given to animal be- 

 havior. Certain forms also afford excellent training in exactness 

 of observation. 



The lack of well defined and unvarying external characteristics 

 makes it difficult to identify many Turbellaria. A large part of 

 the material ordinarily collected is sexually immature whereas, as 

 has been noted above, a knowledge of the structure of the sex 

 organs is necessary in certain genera for identification. Preserved 

 material if immature is especially difficult to identify since the 

 body becomes distorted in shape and the color is usually so 

 modified as to be unrehable. The differences in color and form 

 between several of the species of planarians while definite are 

 so sHght as to be apparent only after a comparison of Hving 

 material. In other cases there is a wide variation in color be- 

 tween individuals of the same species. 



Until comparatively recently descriptions of many species of 

 Turbellaria were extremely meager. The confusion which has 

 arisen as a result is due to the lack of conspicuous external char- 

 acteristics which would serve for identification. 



