TROPIOMETRA CARINATA. 19 



forming a slight elevation; but there is no opening. Apparently there is no 

 outer opening either in the specimen of the same age figured in plate ix, figure 9. 

 Russo {op. cit., pp. 47, 48) maintains that in Antedon the original hydro- 

 pore and pore canal become obliterated 3 to 4 days after the fixation of the 

 larva; 7 to 8 days after the fixation the definitive hydropore and pore canal 

 develop. In Tropiometra I have been unable to ascertain whether this like- 

 wise 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. 



It should be mentioned that in one scries of longitudinal sections of an 

 embryo 10 hours old there is seen a kind of tube or canal visible through 5 

 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 so far as can be judged from the some- 

 what unsatisfactory preservation of the interior of this embryo (the ectoderm 

 and the said tube 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 entoderm and the inner end of the 

 tube. It is, then, hard to see how this could represent a pore canal. In 

 Antedon Seeliger has found the porus in the fully formed larva, not in a stage 

 so young as this. 



Russo does not distinguish between the stone canal and the pore canal, 

 designating both as " canale petroso." He certainly distinguishes 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," 

 this 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 should certainly be upheld clearly. 



In plate viii, figure 2, is seen the opening of the pore canal into the parietal 

 canal, and in plate viii, figure 7, the stone canal is seen to open into the 

 latter. Finally, plate viii, figure 3, shows the parietal canal in direct com- 

 munication with the ccelom. Thus the final arrangement of the interrelations 

 between the hydrocoel and the coelom has been established. But there is 

 still only the one (primary) stone canal and pore canal. This is seen in toto 

 in plate ix, figure 9; but it was impossible to follow the stone canal very 

 distinctly in its entire length in this specimen. 



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



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



now well developed and about to open to the exterior (plate viii, figure 8). 



The epidermis shows a slight invagination over it, but there is still no anal 



opening formed. The place of the future anal opening is seen to be near the 



