496 BULLETIN 82, UNITED STATES NATIONAL MUSEUM. 



On the outer side the epithelium has everywhere given rise to parallel longi- 

 tudinal fibrillae, which in cross section appear as strongly refractive bodies, and 

 viewed longitudinally appear cross-striped. Toward the root the fibrillse extend 

 to the hydroccele ring. On the inner side the fibrillse are here considerably 

 coarser than on the outer, and their distalmost ends stretch over a part of the 

 ring musculature of the hydroccele ring, running between it and the ectoderm. 

 On the thinner outer wall the fibrillse become finer basally, and some (but not all) 

 of them continue as extremely fine fibers, to the oral part of the outer wall of the 

 ring canal. Seeliger was unable to recognize cross striping in the ends of these 

 fibrillae. 



The structure of the hydroccele cavity of the small interradial tentacles is 

 essentially the same. At their bases, however, the outer muscular fibrillse are 

 especially well developed, and can be followed somewhat farther aborally than 

 those of the inner side. The hydroccele of these tentacles is connected with the 

 hydroccele ring by a common base, over which only the outer fibrillse pass. 



There are no ring fibrillse in the tentacles. 



PARIETAL CANAL. 



During this period the parietal canal undergoes no important changes; but 

 the contrast in the histological structure of the two sections becomes more and 

 more emphasized. 



The terminal section is tubular, and is composed of a cubical or fairly high 

 prismatic epithelium, which is probably ciliated. As in earlier stages it runs 

 transversely, and its inner end is depressed more or less steeply toward the aboral 

 and originally right side, reaching about to the vertical mesentery of the right 

 coelome, where it passes over, the walls gradually becoming thinner, into the 

 parietal cavity. The outermost portion of the terminal section turns outwardly 

 at a sharp angle to open as a pore near radius I, and always in the interradius 

 V-I, where the anus also lies. 



The pore varies in form and width and appears to be able to expand and 

 to contract. Since special muscles are absent, the changes in the shape of the 

 pore must be connected with the changes in the entire calyx. 



The end of the canal immediately adjacent to the pore is ciliated, and in 

 some places longitudinal striping can be recognized in the ends of the large 

 cells. If any ectoderm cells at all are involved in its formation they can only 

 be very few in number and at most can only form the outer border. 



The inner section of the parietal canal, the parietal cavity, shows consider- 

 able differences in regard to its form as well as in regard to the histological 

 structure of its walls. Generally speaking, it stretches in a peglike blind extension 

 more or less far toward the aboral pole, lying closely upon the parietal layer of 

 the aboral coelome. Into the broader portion the stone canal opens. 



Sometimes in old larvse the wall can be demonstrated as a distinct pavement 

 epithelium, especially in the aboral end piece; again it becomes fused with the 



