174 BULLETIN OF THE 



assist in changing the water contained in the canal. In the latter func- 

 tion it would be assisted by the cilia lining the canal. 



At its posterior end the canal widens abruptly into a saucer-shaped 

 cavity, which lies with its concave surface npon the antero-ventral face 

 of the brain (Figs. 3, 20, 27), and includes a low rounded prominence 

 (o. ceb.) which I regard as the cerebral organ proper. Macroscopically, 

 this appears to be continuous with the brain, but internally the con- 

 nective-tissue capsule separates it almost entirely from the ganglionic 

 mass. The histological character of this prominence, and its relation to 

 the brain, require more extended consideration. 



When one examines a longitudinal section of this region (Plate III. 

 Fig. 27), perhaps the most striking feature is the extremely prominent 

 cuticula (4 fi in thickness), which covers exactly the convex surface, 

 and only that portion, for at the margin of this convexity (f, Fig. 27) 

 it passes abruptly over into the very thin cuticula of the canal wall. 

 At each lateral edge of the cavity there is a considerable thickening of 

 the cuticula, which extends a short distance into the subjacent tissue 

 and has in cross section the outline of a small retort. The cuticula pre- 

 sents a sharp outer boundary, and there one finds no remnants of cilia 

 in the sections, yet I am inclined to think that cilia are present in the 

 living animal. For in preserved specimens the entire lower portion of 

 this canal is filled with a granular coagulum, which might easily enclose 

 and obliterate cilia, if indeed any were preserved in this deep and nar- 

 row canal, where fluids evidently could not readily penetrate. The lat- 

 eral cilia, which are perfectly distinct in the anterior half of the canal, 

 become gradually less so, until in the lower portion, which is filled with 

 this coagulum, they entirely disappear. In partly macerated specimens 

 this thick cuticula breaks up into small blocks along lines extending 

 perpendicularly to the surface, so that one may reasonably assume that 

 there is a ciliated condition of this surface in the living animal. 



It is difficult to study the cells which underlie this cuticula, inasmuch 

 as the cell boundaries are very indistinct ; the most evident feature is 

 the regular row of nuclei which lies close under the cuticula. From 

 these a crowded mass of nuclei (cl. gn. V) and fibres extend at right an- 

 gles to the surface into an irregular group of fibres (transsected in Fig. 

 21, corns, a.), — the anterior commissure already described. If one ex- 

 amines the nuclei, their resemblance in size, shape, and optical proper- 

 ties to those of the central nervous system is evident. An actual 

 entrance of the fibres into this anterior commissure can also be easily 

 observed. The connection of these fibres and nuclei with the hypodermal 



