40 STUDIES IN THE DEVELOPMENT OF CRINOIDS. 



Crinoids where this process has been observed, Antedon and Tropiometra. 

 This is evident enough from figvire 8, plate xvii, and figure 3, plate xviii. 

 An apparent exception from this would seem to be the embryo from which 

 figures 9 to 12, plate xvii, have been made. Here the invagination appears 

 to be closed in the middle part (figure 11), while it is open posteriorly and 

 anteriorly. Still, this is only an apparent exception. The closure is not 

 real; it is due only to the sides of the invagination lying in this place so close 

 together that it appears quite closed. But there is still a fine vertical line 

 in the middle, indicating that the two side-lobes are in reality separate. 

 However, it would seem that the posterior part of the invagination is closed 

 in the way indicated by these figures, that the side-walls join and coalesce, 

 and that it is only in the anterior part that a covering waU grows upwards, 

 thus closing the invagination, as is evident from figure 8, plate xvii. 



A very conspicuous feature in this process of the closure of the vestibulary 

 invagination in Isometra is the nearly complete obliteration of the lumen of 

 the vestibulum. As seen in plate xvii, figure 8, and plate xviii, figure 3, 

 the walls are joining so closely that merely a line is seen indicating the limit 

 between the epithelium of the covering wall and that of the bottom of the 

 invagination. Later on, the walls must again separate, so that the vestibulum 

 acquires the typical shape (plate xviii, figures 1 and 2) . The last vestige of 

 the invagination is a very narrow opening in the shape of a deep canal at 

 the anterior end, just below the suctorial disk (plate xvii, figure 8). It closes 

 completely while the embryo is stiU lying within the egg-membrane (plate 

 XVIII, figure 3). 



The hydroccel has begun to differentiate, the 5 primary tentacles (or 

 radial canals) having appeared (plate xvi, figures 4 and 8). The stone canal 

 is developing, but has not yet opened into the parietal canal (plate xvii, 

 figure 7). The hydropore is probably still open in the embryo, from which 

 figures 1 to 4, plate xvii, have been drawn, although it could not be 

 discerned beyond doubt in the sections following that represented in figure 4. 

 But in other series, such as that shown in plate xvii, figures 9 to 12, there is 

 certainly no pore. In plate xvi, figure 3, the pore canal can be discerned, 

 but it is impossible to discern an outer opening of it. The result is, then, 

 evidently that the hydropore, which was distinct in the younger stage, 

 has become obliterated or is about to disappear in this stage. The anterior 

 prolongation of the parietal canal is still distinct (plate xvii, figure 8; plate 

 XVIII, figure 3), but apparently beginning to be reduced. In the ccElomic 

 vesicles there is but little change worthy of note. The left or oral ccelom 

 has developed two up'O'ard prolongations at the ventro-lateral side, so that 

 two small spaces appear in the transverse sections laterally to the hydroccel, 

 the whole dorsal side being occupied by the right or aboral ccelom (plate xvii, 

 figures 3, 6, 11, and 12; see also plate xvi, figure 6). The aboral ccelom has 

 acquired a somewhat complicated shape on account of a deep notch in the 



