TROPIOMETRA CARINATA. 13 



soon begins to flatten and at the age of 40 hours has completely assumed an 

 endothelial character. 



It has not been possible to follow the details of the formation of the 

 chambered organ, but plate iv, figure 1, shows the first rudiment of it in the 

 shape of a forward prolongation from the right ccelomic vesicle in an embryo 

 25 hours old. Whether there are 5 such prolongations, as in Antedon, I have 

 been unable to confirm; but there would scarcely be 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. 



The hydrocoel has been completely separated from the entoderm at the 

 age of 16 hours (plate in, figure 5). It sends out a forward prolongation, 

 still in open connection with the hydroccel vesicle (plate in, figure 6). This 

 is the future parietal canal. At the age of 20 hours it is completely separated 

 from the hydroccel. It is only a short vesicle (plate iv, figure 1); I have 

 never found it prolonged anteriorly, as is the case in Antedon (Seeliger, 

 taf. 16, figure 68), and as I have found it also in Compsometra serrata (plate xi, 

 figures 8 and 9), Isometra vivipara (plate xv, figure 11), and Notocrinus virilis 

 (plate xxiv, figure 4). The pore canal does not begin to develop until about 

 the age of 40 hours, and there is as yet no exterior opening (plate v, figures 

 3, 4, and 5). 



Russo (op. cit., p. 47) maintains that the parietal canal in Antedon remains 

 in open connection with the hydroccel. In Tropiometra this is decidedly not 

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

 and Notocrinus virilis. As for Antedon, I would venture to say that Russo's 

 figure 12, tav. n, to which he refers especially as a proof of this statement, 

 does not appear to be a very convincing proof against Seeliger's clear and 

 detailed figures. Certainly the said figure shows a connection between the 

 parietal canal and the hydroccel, 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 hydroccel. 



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

 (plate iv, figure 1), gradually curves and assumes the shape of a horseshoe 

 (plate v, figure 3). The opening is at the left side. The primary tentacles 

 have begun to appear already at the age of 40 hours (plate v, figure 2). 

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



The entoderm remains a simple sac, distinctly dorso-ventrally compressed 

 towards the end of this period (plate v, figure 9). It should be emphasized 

 only that no cells are observed to wander into its lumen. 



4. CLOSING OF THE VESTIBULUM ; FIXATION OF THE LARVA. 

 In accordance with the fact that the fixation may take place at a very 

 different age some larvae attaching themselves at the age of 2 or 3 days, 



