ORGANS OF SPECIAL SENSE 171 



too, have quite irregular contours and which resemble greatl}^ 

 the gland of the pit. 



In addition to this accessory gland there are three non-striated 

 muscle fibers associated with the pit. These arise beneath the 

 dorsal epidermis of the margin of the body of the animal and are 

 inserted laterally upon the syncytial wall of the pit (figs. 2 and 

 7, M). The contraction of these muscles lifts the pit laterally 

 from the substratum. Some of these muscle fibers extend 

 through the gland cell or between its branches. 



This pit, then, with its sensory and glandular regions and its 

 definite musculature does not conform to the conception pre- 

 sented by the following statement from von Graff ('09, S. 64) who 

 says of rhabdocoeles that ''Die Griibchenflecken sind Haut- 

 stellen, die keine Rabdoiden und Drlisenausfiihrungsgange be- 

 sitzen," and likewise Wilhelmi ('08) in his description of the 

 'Auricularsinnesorgane,' gives no such differentiation. However, 

 in a former paper ('12) the authors have shown that the ciliated 

 pits of Microstoma caudatum Leidy were so differentiated and 

 contained both a sensory and a glandular region. Likewise, 

 there were well defined ducts which emptied the secretion of the 

 large unicellular glands into the fundus of the pit. From that 

 study we drew the conclusion that the structure of the ciliated 

 pit of Microstoma caudatum strengthens the affinity between 

 the Turbellaria and the Nemertini. This conclusion was drawn 

 because the 'cerebral organs' of the Nemertini, in their simplest 

 condition, were, until our study of Microstoma, supposed to 

 differ from the ciliated pits of the Turbellaria only in that they 

 were differentiated into a sensory and a glandular region, while 

 the ciliated pits were not so modified. 



The present observations reveal the fact that there is in 

 Prorhynchus applanatus a ciliated pit which is very much more 

 highly specialized, both in its cytology and possession of a defi- 

 nite musculature, than that of Microstoma caudatum. There- 

 fore Prorhynchus applanatus carries us a step further in estab- 

 lishing the affinity between the Turbellaria and Nemertini. 



