DEVELOPMENT OF PARA VORTEX GEMELLIPARA 465 



suggested that negative reaction to light brought about this 

 result, but as explained above it is extremely unlikely that the 

 adults are thus negatively reactive to daylight. The marked 

 positive reaction of the young specimens is probably responsible 

 to some extent for their leaving the host within which they 

 develop and so gives them an opportunity to enter other mus- 

 sels. In the fulfillment of this end the negative geotropism 

 of these young worms may also be of service; it would tend to 

 cause a migration upward toward the source of light. 



When removed from their hosts the healthy worms of medium 

 size glide along the bottom of the dish at the rate of about one 

 millimeter per second. Linton noted that the course in one 

 direction was not long held, but often changed from one side 

 to the other following a turning movement of the anterior end 

 of the animal. Frequent Ij^ these older worms halt and turn 

 rapidly about for several revolutions in a circle whose diameter 

 does not exceed the length of the individual (fig. 2). Hallez 

 noted this circus movement as being extremely frequent in the 

 case of Paravortex cardii. 



The young worms move with greater rapidity for their size and 

 in a much more direct course than the older invididuals. Further- 

 more they follow one direction often for several inches, only 

 deviating in order to avoid obstacles. As already stated this 

 course is consistently toward the light source. Rarely does a 

 young worm perform the circus movement. 



It is interesting to note in passing that Paravortex gemellipara 

 like other Turbellaria observed by von Graff, is itself subject 

 to parasitism. Frequently while studying living worms under 

 the microscope numbers of an hypotrichous Infusorian are 

 seen moving over them. One worm was so completely covered 

 with them that, when the cover slip was pressed, these Infusoria 

 became separated in such a way as to give the impression that 

 the epithelium of the worm was peeling off. The hypotrichs, being 

 cihated only on one side, and closely approximating the epithelial 

 cells in size, appeared strikingly similar to them. 



