BY VERA A. IRWIX-SMITH. 763 



cliloroform. In these cases, the handling of the minute speci- 

 mens was a great difficulty. In all cases, I found it necessary 

 to stain the worm slightly before embedding, in order to locate 

 it with the dissecting microscope when cutting out the celloidin 

 or paraffin block. 



The mounted sections were afterwards double-stained with 

 Erlich-hsematoxylin and eosin. Satisfactory sections are difficult 

 to obtain. C/uetosoma haswelli is so delicate, that the body- 

 walls tend to fall together during the embedding process; and, 

 when this occurs, the transverse sections take the form of a 

 narrow thread, in which it is impossible to make out details of 

 internal structure. Chcetosoma falcatum and NolocJicpiosoma 

 tenax, which have a very thick, external cuticle, and, conse- 

 quently, a firmer outline, sometimes give good transverse sec- 

 tions, but they tend to tear out of the block, especially in longi- 

 tudinal section. 



For many of the specimens examined, I am indebted to Pro- 

 fessor Haswell, whom I have to thank for the direction of the 

 work, and in whose laboratory it was carried out. 



I have also to thank Acting-Professor S. J. Johnston for the 

 valuable assistance he has given me in the preparation, both 

 of the material and the paper; and Mr. R. J. Tillyard for his 

 kindly and helpful criticism and interest during the progress of 

 the work. 



Structural Features of the CHiETOsoMATiD^. 



All the Chcelosomatidce have an elongated, cylindrical, worm- 

 like body, tapering posteriorly to a sharply pointed tail, and 

 usually swollen anteriorly to form a more or less distinct head. 



For purposes of classification into genera aiid species, the dis- 

 tinguishing features are the shape and size of the head-swell inf', 

 and the extent of its demarcation from the trunk; the character 

 of the pharynx; the number of rows of ventral and head-seta?, 

 and their position; the character of the setae; the relative prp- 

 portions of the trunk; the thickness of the external, cuticular 

 covering; the variations in the character of the transverse stria- 

 tions, and in the size and arrangement of the body-hairs with 



