44 STUDIES IN THE DEVELOPMENT OF CRINOIDS. 



found likewise in the other parts of the young Pentacrinoid, although in 

 much smaller numbers. This was also found to be the case in Tropiometra 

 (see above, p. 17). Of course, the definite proof that this is the correct 

 interpretation can not be given here, the insufficient preservation making 

 such minute histological researches impossible. But it seems to me that all 

 the facts point in this direction, that a histolysis of the entoderm takes 

 place during the metamorphosis, the wandering cells taking an active part 

 in this process. 



With the replacement of the vestibulum from the ventral side of the 

 embryo to its posterior end, the hydrocoel and the ccelomic spaces have also 

 occupied their final position (plate xviii, figures 1 and 2). The inner wall 

 of the vestibulum is very thick, with many of the grains just mentioned 

 lying irregularly, in places more scattered, in others in close groups between 

 the nuclei. A slight concavity in the middle (plate xviii, figiu-e 2) is the first 

 indication of the future esophagus; below this place the ectoderm and 

 entoderm are fusing in the middle of the still open hydrocoel ring. 



The stone canal has not yet opened into the parietal canal and the pore 

 canal has no exterior opening. The figures (plate xix, 1 to 4) show the inter- 

 relations of these structures very distinctly. In the mesentery inside the 

 parietal canal a sfight accumulation of nuclei is seen; it probably represents 

 Russo's primary gonad. 



A somewhat more advanced stage is represented in plate xxi, figure 2, 

 and in the sections, plate xviii, figures 4 to 6. The tentacles have protruded 

 into the vestibulum, which is about to open, a depression having appeared 

 in the middle of its outer wall and the 5 oral valves being about to separate. 

 The mouth and esophagus have been formed and in the stomach a lumen 

 is beginning to appear, the granular mass being about to be absorbed (plate 

 XVIII, figiu-e 5). Also the intestine has been differentiated, but the anal 

 opening has not yet been formed. The stone canal has opened into the pari- 

 etal canal (plate xviii, figure 4), but no outer opening of the pore canal can be 

 discerned as yet. The chambered organ is beginning to assume its typical 

 form and the axial organ is distinct (plate xviii, figiu-e 6). It appears 

 that the parietal canal has opened into the oral coelom; but this could not 

 be fully ascertained. 



The next important change is the opening up of the vestibulum. The 

 young Pentacrinoid now begins to feed directly, having till now subsisted 

 on the yolk substance contained in the egg. Diatoms, mainly of the Coscino- 

 discus forms, are found in the stomach, and in some cases also larva? of its 

 own kind, as mentioned above (plate xix, figure 5). I have foimd very young, 

 just opened, Pentacrinoids with such a larva hanging haK out of its mouth, 

 it being too large to be swallowed. 



In plate xxi, figiu-es 3 and 6, are represented two Pentacrinoids of some- 

 what more advanced stages, with the arms already branching. They have 



