CILIATED PITS OF PRORHYNCHUS STAGNALIS. 189 



(Figs. IV, and VI, r.), presumably for the receiving of stimuli. 

 These ridges are devoid of cilia, though the remaining inner 

 surface of the pit is highly ciliated. Due to the difficulty in 

 differentiating nervous tissue very clearly through any of the 

 simpler staining methods, it was impossible to distinguish any 

 special nerve endings. The main anterior nerves from the dorsal 

 ganglia were closely applied to the outer surface of the pit along 

 its mesial side only (Figs. IV, V, VI, and VII, a.n.). Until better 

 stained sections can be studied, the matter of nerve application 

 to the pit will have to rest with this general statement. 



At the base of the lumen of the pit, and connected with it only 

 through small pores (p.] which pass through the syncytial wall of 

 the pit on the anterior angle of the fundus lies'a large uni-cellular 

 gland (Figs. IV, V, VI, and VII, g.}. Its contour is very irregular, 

 the cell giving off a number of processes. There are two inter- 

 esting features to this glandular cell not noted in that of Pro- 

 rhyncJms applanatus. One concerns the characteristic bent, 

 ovate contour of the nucleus of this cell (Figs. IV, V, and VII, 

 g.n.), with clearly denned chromatin granules. The other 

 interesting fact concerns the presence of a simple drainage system 

 in the form of intra-cytoplasmic canaliculi (Figs. IV, V, VI, and 

 VII, en.), which pour the mucus-like secretions of the cell through 

 the pores in the syncytial wall into the pit (Figs. IV, VI, and VII, 

 p.). These canaliculi have been observed both in an empty 

 state and filled with materials of secretion. 



Connected with the ciliated pits are several non-striated muscle 

 fibers, arising both laterally and mesially, and passing closely 

 along the surface of the glandular cell. There is only one fiber 

 attached to the lateral wall (Figs. IV and VII, l.m.), but at least 

 three fibers attached to the mesial wall of the pit (Figs. VI and 



VII, m.m.). 



SUMMARY. 



1. The correlation between the position of the ciliated pits in 

 Prorhynchus stagnalis and the way in which the anterior portion 

 of the body makes its exploratory movements is met in this 

 rhabdoccele in the same manner as in Microstoma caudatum and 

 Prorhynchus applanatus. 



2. The ciliated pits of Prorhynchus stagnalis are composed of a 



