414 University of California Publications in Zoology. [VOL. 6 



acetabulum. The circular margin of this opening is rounded and 

 thick, due to the presence of a heavy sphincter. Surrounding 

 this opening, within the sphincter of the margin, is the anterior 

 excretory ring and the anterior nerve ring. This is formed as 

 described by dorsal and ventral terminal branches of the anterior 

 lateral stems, united in the median line by a dorsal and ventral 

 anastomosis. This ring is thus composed laterally of a single 

 large nerve, but in the median line of many small anastomosing 

 threads (pi. 39, fig. 42). In its lateral portions at the points of 

 junction with the lateral nerve stem, there is a slight enlarge- 

 ment in which typical ganglion cells are present. From the ring 

 are given off nerves ramifying in the margin of the acetabular 

 opening, and in the outer layer of the acetabular muscle coat. 

 The ring also receives the peripheral stems mentioned above. 



The Central Nervous System of the Rosette and Canal. The 

 posterior rosette may be considered as composed of two parts, 

 a funnel and a canal. The wide-mouthed funnel with glandular, 

 much folded walls is circular in cross-section, and roughly V- 

 shaped in sagittal section, the point of the V being directed 

 anteriorly. That is to say, the course of the funnel tube is almost 

 straight forward through the center of the body (pi. 46, fig. 76). 

 Its posterior margin is bordered by the frills which make up the 

 posterior rosette. At the base of these frills there is a thickening 

 of the walls of the funnel commonly referred to as the neck of 

 the rosette, due to the formation of a sphincter by the inner 

 transverse muscle. This funnel leads into the canal, a narrow, 

 non-glandular region, passing a little anteriorly and almost 

 directly ventrad to open on the ventral surface of the valve-like 

 canal opening. The region where the canal joins the funnel, 

 the apex of the V, will be called the tope, from the corresponding 

 region of an ordinary funnel. The walls of the canal itself show 

 no central innervation. There are apparent fine fibres of the 

 intermediate and cuticular nerves but no branches of the central 

 system. From the tope of the funnel posteriorly the walls of the 

 passage are weakly innervated by a complicated set of commis- 

 sures, stems, and branches, all belonging to the central system. 

 consisting of the following parts : 



1. A posterior bridge commissure connecting laterally situ- 

 ated ganglionic knots. 



