312 BULLETIN 82, UNITED STATES NATIONAL MUSEUM 



hydrocoel ring has assumed an endothelial character, and numerous trabeculae 

 cross its lumen. Two pairs of tentacles have developed at the side of each primary 

 tentacle. The outer of these pairs represents the interradial tentacles. In the 

 sections through the vestibulum the tentacles are seen to be separated from one another 

 by a fine but distinct 1m e which probably indicates the presence within the vestibulum 

 of a slimy fluid. 



The communication between the hydrocoel and the exterior has at last become 

 established. Russo wrote that in Antedon the original hydropore and pore canal 

 become obliterated three to four days after the fixation of the larva; from seven to 

 eight days after the fixation the definitive hydropore and pore canal develop. In 

 Tropiometra Mortensen was unable to ascertain whether this likewise takes place, 

 not having sufficient material of the later stages of the pentacrinoids ; but in any case 

 there has been no exterior opening before this stage. 



Mortensen said it should be mentioned that in one series of longitudinal sections 

 of an embryo ten hours old there is seen a kind of tube or canal visible through five 

 consecutive sections. With its outer end it forces itself to some degree in between the 

 ectoderm cells, pushing their basal ends aside. But it does not reach the surface, 

 and there is no outer opening. One might be inclined to see in this a primary pore 

 canal, but as far as can be judged from the somewhat unsatisfactory preservation of 

 the interior of this embryo (the ectoderm and the tube under consideration are very 

 well preserved) the entoderm is in a very primitive stage of development, not yet 

 separated into its two main parts, and there is, of course, no trace of the parietal canal 

 as yet. Furthermore, it is impossible to discover any connection between the ento- 

 derm and the inner end of the tube. It is therefore difficult to see how this could 

 represent a pore canal. In Antedon Seeliger found the pore in the fully formed larva, 

 not in a stage so young as this. 



Mortensen pointed out that Russo did not distinguish between the stone canal 

 and the pore canal, designating both as "canale petroso." He certainly distinguished 

 the "canale petroso interno o primario" and the "canale petroso esterno o secondario," 

 but as, according to Russo, there is also a primary external "canale petroso," the 

 nomenclature is confusing. The distinction between the stone canal and the pore 

 canal is of great morphological importance, as is well emphasized by Bury, and Mor- 

 tensen rightly maintains that it should certainly be clearly upheld. 



The pore canal opens into the parietal canal, and the stone canal opens into the 

 latter. The parietal canal is in direct communication with the coelom. Thus the 

 final arrangement of the interrelations between the hydrocoel and the coelom have 

 become established. But there is still only the one (primary) stone canal and pore 

 canal. 



The oral coelom appears to be in open communication with the aboral coelom. 



The stomach is empty, showing no cells in its lumen. The rectum is now well 

 developed and about to open to the exterior. The epidermis shows a slight invagina- 

 tion over it, but there is still no anal opening formed. The place of the future anal 

 opening is seen to be near the adjoining radius, not in the middle of the interradius. 

 When it has been formed the young crinoid is ready to open the vestibulum, and the 

 embryonic life proper is ended. 



The first rudiments of the skeleton appear at the age of 24 hours, when the basals 



