384 



II [RüSIII OHSHIMA : 



The posterior end of the midventral raidial canal next dilates 

 transversely (Textfig. F, mvr). Corresponding to each lateral end of 

 this cavity the ectoderm forms a pit, the rudiment of the primary 

 pair of pedicels (Pl. V, figs. 3-6, //> and rf). Either of them may be 

 anterior to the other in position, differing in this respect from the cases 

 observed by LUDWIG 1 and NewTH. 2 In erosi section each pedicel lies 

 at an angle of ca. 40 with respect to the midventral line. The dorsal 

 pore is in many cases slightly shifted to the right side. The two 

 pedicels as well as the dorsal pore are all situated between the second 

 and third ciliary bands. The stomodajum lies in front of the first 

 ciliary band, and in cross section atout 30 to the left of the mid- 

 ventral line, and leads into a spacious atrial cavity into which the 

 primary tentacles project. At about this time the four remaining radial 

 canals arise from the hydrocœle-ring as at first anteriorly directed buds, 

 but soon grows outwards and turns posteriorly. Of these, the right 

 dorsal one (Textfig. F, Pl. V, fig. 6, rdr) appears first, and the left 

 dorsal one (Idr) next. The fifth tentacle (Textfigs. F and G, Pl. V, 

 figs. 4 and 6, 4) now makes its appearance in the left dorsal inter- 

 radius, and the right ventral radial canal (rvr) soon afterwards. 



The peculiar relation of the five primary tentacles to the ra üal 

 canals pointed out by LUDWIG 3 holds true in this case as well (Text- 

 fig. G, Pl. V, fig. 6). But it is not quite clear in the early stages, 

 where the rudiments of the tentacles and the radial canals appear to 

 arise directly from the ring-canal independently of each other (Textfig. 

 E, Pl. V, fig. 5). 



The time of closure of the horse-shoe shaped hydrocœle in'.o a ring 

 varies somewhat. It may occur before the appearance of the two 

 lateral ventral radial canals, but more often after the formation of the 

 five radial canals and the Polian vesicle. In the latter case the left 



1) H. Ludwig, Sb. k. pr. Ak. Wiss., 1891. 



2) H. G. Newth, P. Z. S., 1916. 



3) II. Ludwig, Sb. k. pr. Ak. Wiss., 1891. 



