308 BULLETIN 82, UNITED STATES NATIONAL MUSEUM 



terior end, below the apical tuft, is seen a conspicuous layer of an exceedingly fine 

 fibrillar substance which continues more or less distinctly downwards, below the 

 vibratile bands. The nerve passing along the side of the vestibulary invagination 

 may also be observed, but not so distinctly as appears to be the case in Antedon. 

 Unfortunately the preservation is not good enough to permit the determination of its 

 nervous character from the histological structure, but from the complete analogy of 

 its position with the nervous system of the Antedon larva it would appear beyond 

 doubt that this structure in the Tropiometra larva represents the nervous system. 



In embryos 16 hours old the right and left coelomic vesicles have separated com- 

 pletely and are about to assume their final position, the left at the posterior end of 

 the embryo, the right along the dorsal side covering the entoderm. The right vesicle 

 widens gradually, so that at the age of forty hours it occupies the whole dorsal side 

 in the posterior half of the embryo. The epithelium of the vesicles soon begins to 

 flatten, and at the age of forty hours has completely assumed an endothelial character. 



Mortensen says that it was not possible to follow the details of the formation of 

 the chambered organ, but in an embryo twenty-five hours old the first rudiment of 

 it is seen in the shape of a forward prolongation from the right coelomic vesicle. 

 Whether there are five such prolongations, as in Antedon, Mortensen was unable to 

 determine; but he remarks there would be scarcely any reason to doubt that the 

 development of a structure so fundamental in crinoid anatomy as the chambered 

 organ must proceed in the same way, at least in the uniform group of the comatulids. 



At the age of 16 hours the hydrocoel has been completely separated from the 

 entoderm. It sends out a forward prolongation, still in open connection with the 

 hydrocoel vesicle. This is the future parietal canal. At the age of twenty hours it 

 is completely separated from the hydrocoel. It is only a short vesicle. Mortensen 

 never found it prolonged anteriorly as is the case in Antedon, and as he also found it 

 in Compsometra serrata, Isometra vivipara, and Notocrinus virilis. The pore canal 

 does not begin to develop until about the age of forty hours, and there is as yet no 

 exterior opening. 



Mortensen says that Russo maintains that the parietal canal remains in open 

 connection with the hydrocoel. In Tropiometra this is decidedly not the case, and 

 the same holds good for Compsometra serrata, Isometra vivipara, and Notocrinus 

 virilis. As for Antedon, Mortensen remarked that Russo's figure to which he refers 

 especially as a proof of his statement does not appear to him to be a very clear and 

 convincing proof in the face of Seeliger's clear and detailed figures. Certainly the 

 figure shows a connection between the parietal canal and the bydrocoel, but it would 

 appear to be the stone canal which is here seen to open into the parietal canal, and 

 that does not prove that the parietal canal was always in connection with the 

 hydrocoel. 



The hydrocoel, which at first is a simple vesicle, sometimes very wide, gradually 

 curves and assumes the shape of a horse-shoe. The opening is at the left side. The 

 primary tentacles have begun to appear already at the age of forty hours. There is 

 still no trace of a stone canal to be seen at this stage. 



The entoderm remains a simple sack, distinctly dorsoventrally compressed toward 

 the end of this period. It should be emphasized only that no cells are observed to 

 wander into its lumen. 



