290 CllESSWELL SHEARER. 



The nephridial canals are remarkably difficult to recognise 

 in sections on account of the retraction they invariably 

 undergo during fixing. It is impossible to trace them with 

 any degree of certainty through consecutive sections. For 

 this reason I have relied mainly in rny investigation of the 

 nephridia on methyl-blue impregnation preparations of living 

 material. The figures accompanying the present paper there- 

 fore represent the appearance of the nephridia in living 

 material. It is necessary to use the highest powers of the 

 microscope to determine the structure of the nephridia, and 

 even then the eye requires considerable practice and training 

 to distinguish the motion of their cilia. It is difficult to 

 convey any idea of the extreme delicacy and minuteness of 

 these structures. The necessity of being compelled to use 

 immersion-lenses for their study excludes the use of any of 

 the ordinary dark ground systems of illumination. Doubtless 

 these would offer an excellent means of investigating struc- 

 tures of this nature in an animal so transparent as His- 

 triobdella, if they could be used successfully with the 

 immersion-lens. 



Of great service in the study of the methyl-blue pre- 

 parations is, I have found, the use of a number of sodium 

 glass screens of different shades, such as are used in ortho- 

 chromatic photography to vary the exposure from five to 

 fifteen times. 



To obtain a uniformly constant light I have used an 

 ordinary Welsbacli gas lamp, with standard screens. This 

 gives a light much superior. to that of ordinary daylight 

 in bringing out the finer structure of the nephridial canals. 



For sections I have used the ordinary paraffin and the 

 paraffin celloidin method. The sections were cut of the 

 uniform thickness of 7 /u. In the reconstructions of the nervous 

 system shown in figures I have used a method which is in 

 part a modification ^ of that described by Woodworth (' Zeit. 

 f. wiss. Mik.,' xiv, 1897, p. 15). Each section, of which there 



' This I owe to my friend. Mr. E. W. Nelson, of the Murine Biological 

 Association, Plymoutli. 



