18 JOHNS HOPKINS UNIVERSITY MORPHOLOGICAL MONOGRAPHS. 



walls of the stomach pocket approaching the outer at two points, leaving 

 between them a concavity freely open to the rest of the stomach pocket 

 above and at the sides. Fig. 13, a little lower down, shows the two walls 

 fused together at two points, making the interspaces a definite canal 

 communicating with the stomach pocket above only. This canal lies 

 directly over the sensory niche, and in the next figure (No. 14) the canal 

 is seen to have passed through the roof of the sensory niche and to have 

 entered the base of the stalk of the sensory club. In the enlarged end of 

 the club, the part which bears the sensory structure, the canal widens 

 into a terminal ampulla-like sac. 



The endoderm lining the canal of the sensory club is specially differ- 

 entiated. In the stalk it is more columnar than the epithelium of the 

 stomach pockets, and is made up of cells containing a brightly staining 

 nucleus with very little trace of cytoplasm. The cell bodies appear as if 

 filled with a clear, non-staining fluid. Perhaps these cells give the stalk 

 elasticity to act in connection with the thin layer of longitudinal muscle- 

 fibres that are found just external to the supporting lamella. The epithe- 

 lium of the terminal enlargement of the canal is composed of very high 

 narrow cells, many of which show two nuclei of equal size and staining 

 quality lying side by side. 



In continuation of the specialized epithelium of the perradial fur- 

 rows in the floor of the stomach the inner wall of the stomach pocket 

 shows a strip of similar densely crowded columnar cells leading from 

 the gastric ostium downwards to the canal of the sensory club. As in 

 the other case, the strip probably represents a specially ciliated tract, and 

 perhaps in it we see the reason why the canal of the sensory club is 

 almost always found to contain either spermatozoa which are shed by 

 the male reproductive organs directly into the stomach pocket, or else 

 floating cells of the kind to be described in the next section. 



The canals of the interradial tentacles arise from the peripheral 

 gastro-vascular system much lower down than those of the sensory 

 clubs, since these tentacles have preserved their primary positions with 

 reference to the bell margin. Figure 16 represents a section taken at the 

 level of the base of the pedal ia which gives the connection of the tentacle 

 canals with the gastro-vascular system. At the level below the sensory 

 niche the four broad stomach pockets have been divided, as we have seen, 

 into the right marginal pockets (mp). The figure shows that in the 

 interradial corners the longitudinal septa (ivl, in the preceding figures), 

 or lines of fusion between the two walls of the peripheral gastro-vascular 



