i9o8.] 



J. Stephenson : The Anatomy of some Aquatic Oligochœta. 



271 



This '^opaque body" may , be extruded uninjured from animals which are 

 beginning to break up under examination in consequence of the drying up of the 

 water or pressure of the cover-glass ; it may also be extracted from the animal by 

 manipulation with needles. Its substance consists^ as in the case of Nais variabilis, 

 of oval or slightly irregular highly refractile structureless particles. 



In two specimens modified genital setcB were seen. In one case they were on the 

 sixth segment, two in number on each side, and of the .shape shown in fig. 33. In 

 the other, the setae of the sixth segment were not modified, but those of the fourth, 

 though of the usual type, were extraordinarily massive, being slightly longer and 

 twice as thick as the normal form ; the forking of the distal end was much more 

 unequal, and the curve sharper, than usual (fig. 34) : it is, however, possibly incorrect 

 to call these genital setae. Both specimens were well advanced as regards the 

 development of the genital organs, and resembled the stage shown in fig. 39. 



Variations met with in the genus Pristina. 



I wish here to describe two specimens, one of P. longiseta and one of P. cequiseta 

 which- differ from the normal individuals in the greater or less length of the 

 prostomium. 



Text-fig, 4 shows the specimen of P. longiseta. The Stylaria-M^çt elongation 



Pig. 4. — Sketch of a specimen of Pristina longiseta with abnormally elongated proboscis-like prostomium. 



of the prostomium is obvious ; but the species of the specimen can hardly be 

 doubtful, in view of the presence of dorsal hair-setae on all segments behind the 

 first (they have, however, fallen out in the seventh), the great elongation of those of 



