90 STUDIES IN THE DEVELOPMENT OF CRINOIDS. 



Fio. 6. Frontal, longitudinal section of a fully formed embryo, showing the beginning specialization of the 

 cntoderm, which is without a lumen, being completely filled with yolk-cella in which larger 

 groups of yolk-globules are seen. The upward prolongations of the oral ccdom are seen. The 

 ciliated bands are distinct. 



Fio. 7. Corresponding section from a slightly younger embryo. The outline has been slightly corrected. 



Fio. 8. Part of frontal longitudinal section, showing the primary lobes of the hydrocoel. 



FIG. 9. Sagittal longitudinal section, somewhat oblique, which accounts for the suctorial disk not being seen 

 in the section; showing the lobes from the aboral cci'lom; the outer one of them represents 

 part of the chambered organ. The arrangement of the glandular cells in the vestibulum 

 is to be noticed. 



PLATE XVII. 



(All figures represent Isomctra vivipara. All, except figure 5, X165.) 



FIGS. 1, 2, 3, 4. From a transverse series of a fully formed embryo, figure 1 being the more anterior, figure 4 

 the more posterior of the sections. There are twelve sections between figures 1 and 2, four 

 sections between figures 2 and 3, and twelve sections between figures 3 and 4. In this series 

 the vestibulum is more than usually wide. The difference in the relative position of the gland- 

 ular and nuclear parts of the vestibulary epithelium from the anterior to the posterior part 

 of the vestibulum is to be noticed. 

 Figure 1 shows the chambered organ, the aboral ccclom, divided into two parts by the vertical 



mesentery and the upper part of the parietal canal. 



Figure 2 is below the vertical mesentery, the aboral ccelom being then undivided. The hydroocel 

 is slightly prolonged towards the pore canal, probably the first indication of the stone canal. 

 In figure 3 the forward prolongations of the oral ccelom are seen. 



In figure 4 the oral cceloin occupies the whole space round the dorsal side of the entoderm. The 

 pore canal is seen close to the surface, but no opening is discernible in the following sections. 



FIG. 5. Part of the epithelium within the vestibulary invagination, showing the structure of the cuticula. 

 Only a few of the outer nuclei have been drawn. In the right end of the figure is seen a 

 glandular cell. X750. 



FIQS. 6, 7. From a series of transverse sections; ten sections lie between the two figures, figure 6 being the 

 more posterior. The arrangement of the glandular and nuclear portion of the ectoderm in 

 the vestibulary invagination is to be noticed. In figure 6 the forward prolongations from the 

 oral ccelom are seen. In figure 7 the dorsal or aboral ccelom is divided in two parts by the 

 vertical mesentery. The stone canal is seen in this section, as is also the chambered organ. 



FIG. 8. Sagittal, median longitudinal section, showing the vestibulum nearly closed, only a very narrow 

 opening, discernible only in two sections, still remaining. Above the opening is seen the 

 suctorial disk. The large space above the hydroccel is the parietal canal. The chambered 

 organ has been cut only in the lower part; the horizontal spaces above it indicate the dissolved 

 stalk-joints. The figure has been slightly combined from two sections. It is to be noted that 

 there is no indication of an apical tuft of cilia above the suctorial disk. 



FIGS. 9, 10, 11, 12. From a series of transverse sections. There are twenty sections between figures 9 and 

 10, one section between figures 10 and 11, and seven sections between figures 11 and 12. Fig- 

 ure 9 is the foremost of them. The figures show the considerable difference in the depth of the 

 vestibulary invagination in the anterior and posterior part; the narrowness of the vestibulum 

 is noticeable. In figures 10 and 11 it appears to be closed, but is not really so; the two side- 

 walls have joined very closely, but there is still seen a median line separating them. In figure 

 10 the beginning of the stone canal is seen; in figure 11 the first indication of the axial organ 

 is seen. 



PLATE XVIII. 



(All figures of Isometra vivipara.) 

 FIGS. 1, 2. Longitudinal sections of a newly attached Pentacrinoid. The stomach is filled by a granular 



mass, consisting of phagocytes; the entodermal nuclei lie in a single layer close to the surface. 



The vestibulum has a wide lumen; in the middle of the thick lower wall a slight depression 



(figure 2) indicates the place of the future mouth opening. Seventeen sections between figuree 



1 and 2. X165. 



FIG. 3. Sagittal, longitudinal section of a fully formed larva with the vestibulum closed. X 165. 

 FIG. 4. Opening of the stone canal into the parietal canal; from a longitudinal section of the same series 



as figures 5 and 6. X290. 

 FIGS. 5, 6. Longitudinal sections from a Pentacrinoid with the vestibulum still closed. The tentacles have 



protruded into the vestibulum; the mouth and esophagus have been formed. In figure 5 the 



stone canal is seen in its connection with the hydrocoel ring. There are ten sections between the 



two figures. X 105. 

 FIG. 7. Transverse section of a newly attached Pentacrinoid, corresponding to figures 1 and 2, showing the 



granular mass (phagocytes) filling the stomach. The formation of the rectum and anal opening 



is indicated. X1G5. 



FIG. 8. Sagittal, longitudinal section of fully formed larva, with vestibulary invagination still open. X165. 

 Fio. 9. Section of the suctorial disk of a fully formed larva. X535. 



