1 86 MARGARET R. HAASE. 



For histological study, specimens were fixed with greatest 

 success in Bouin's fluid, cut from five microns to seven microns 

 thick, and stained in iron haematoxylin, with Bordeau red or 

 eosin as a counter stain. It was found best to handle the 

 specimens in very small numbers at a time. While fixing, it was 

 also found advantageous to keep the animal under the slight 

 pressure of a cover glass to prevent the curling of the body. The 

 best stain for differentiating nervous tissues was Mallory's 

 connective-tissue stain, fixation being in Zenker's fluid. The 

 nervous tissue is stained blue in this case. 



Acknowledgment must be made to Dr. W. H. Taliaferro for a 

 box of slides of Prorhynchus stagnalis which he left with Dr. 

 Kepner, and which were of great aid in this work. 



It is necessary to consider the central nervous system with 

 which the two ciliated pits are associated before discussing the 

 latter. There are two dorsal ganglia just anterior to the opening 

 of the muscular pharynx as it lies at rest within the pharyngeal 

 sheath (Figs. II and III, d.g.). These ganglia are connected 

 across the pharyngeal sac by a relatively broad commissure 

 (Figs. II and III, d.c.}. Posteriorly from the dorsal ganglia 

 extend latero-ventrally two nerves, which cannot be traced 

 beyond the beginning of the enteron (Figs. II and III, p.n.~). 

 Anteriorly from the dorsal ganglia extend latero-ventrally two 

 other nerves, which lie in contact with the mesial surfaces of the 

 ciliated pits, and passing beyond the ciliated pits branch in the 

 extreme anterior portion of the body (Figs. II and III, a.n.}. 

 There is another set of nerves connected with the pharynx, the 

 exact path and relation of these nerves to the ganglia and 

 pharynx having not been definitely determined. 



From Kepner and Taliaferro's ('16) paper on "Organs of 

 Special Sense of Prorhynchus applanatus Kennel," we find this 

 comparison of the ciliated pits in Microstoma caudatum and 

 Prorhynchus applanatus: 



"As stated before, we consider the ciliated pits the chief organs 

 of special sense. These organs open on the ventral side and are 

 disposed laterally and ventrally to the thick nerve commissure 

 which joins the two dorsal ganglia. . . . When the animal is 

 crawling upon a surface, it seems to make numerous exploratory 



