10 ILLINOIS BIOLOGICAL MONOGRAPHS [456 



other portion was teased apart to free the larvae for preserving for 

 toto mounts, and the remainder was used for the study of the living 

 animals. Much of the anatomy of the cercariae could be made out 

 from living specimens. In fact this proved to be an extremely impor- 

 tant part of the study, since some points, for example the smaller 

 branches of the excretory system and the movement, could only be 

 observed in this manner. 



For preservation of material a number of fixatives were used. In 

 fixing the freed cercariae for toto mounts the best results were obtained 

 by the use of hot solutions of Bouin's picro-aceto-formol or corrosive- 

 acetic. For sectioning the infected organs were fixed in toto, since as 

 suggested by Cary (1909:597) this is easier and gives better results 

 than attempting to section freed individuals. A corrosive-acetic solu- 

 tion was ordinarily used for fixing this material. Rediae and cercariae 

 for toto mounts were stained in Mayer's haemalum, Delafield's haema- 

 toxylin, Conklin's picro-haematoxylin, and Mayer's paracarmine. The 

 specimens were as a rule considerably overstained in dilute solutions 

 of the stain and differentiated in HC1 2 under the microscope. In 

 mounting it was found convenient to place large numbers of the larvae 

 on one slide. The infected organs to be sectioned were usually stained 

 in bulk in Ehrlich's acid haematoxylin, cut into sections 5 to 7 micra 

 in thickness, and differentiated on the slide, part of them being coun- 

 terstained in eosin. 



On account of their great mobility, small size, and remarkable 

 power of changing their shapes, some cercariae are very difficult objects 

 to study. No very accurate measurements can be made of living speci- 

 mens, and in preserved material they are often contracted and dis- 

 torted. With the living cercariae an attempt was made to get the 

 range of variability, and in preserved material, whenever possible, the 

 average measurements of a number of well extended specimens were 

 taken. The measurements of preserved material are less than those 

 from living specimens of the same kind, even the suckers shrinking 

 perceptibly after preservation. For these reasons comparisons of the 

 cercariae based on size and shape, as Liihe (1909:173) has suggested, 

 are not always by themselves very reliable criteria for specific deter- 

 mination. 



