8. ILLINOIS BIOLOGICAL MONOGRAPHS [288 



All the forms described in tliis paper were studied as toto mounts; 

 where sufficient material was available sections were made, and many 

 were studied alive. The importance of the study of the living specimens 

 can not be overemphasized as the best method of tracing the excretory 

 system. Also, by observing the living animal as it moves, it is possible 

 to measure the extent of normal variation in form that occurs in a single 

 specimen as different shapes are assumed concomitant with the move- 

 m.ents of the animal ; in forms with such soft bodies and variable shapes, 

 a study of preserved material alone has in many eases given false con- 

 ceptions concerning morphological relationships of organs and systems. 

 In toto mounts a support under the eoverglass is necessary to prevent it 

 from flattening and distorting the normal shape of the aspidogastrids 

 and to avoid crushing the caudal disc of the poly.stomes. For the stain- 

 ing of specimens to be mounted in toto, better results were obtained by 

 the use of carmine than by hematoxylin stains. For staining sections 

 the method that proved most valuable was to use the hematoxylin stains 

 for differentiating the nuclear elements and various plasma stains for 

 counterstaining. 



