90 STUDIES IN THE DEVELOPMENT OF CRINOIDS. 



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

 entoderm, which is without a lumen, being completely tilled with yolk-cells in which larger 

 groups of yolk-globules are seen. The upward prolongations of the oral ca?lom are seen. The 

 ciliated bands are distinct. 



Fig. 7. Corresponding section from a slightly yoimger embryo. The outline has been slightly corrected. 



Fig. 8. Part of frontal longitudinal section, showing the primary lobes of the hydroca'l. 



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

 in the section; showing the lobes from the aboral coclom; 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 Isomelra i-inpara. 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 jjosterior 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 coelom, divided into two parts by the vertical 

 mesentery and the upper part of the parietal canal. 



Figure 2 is below the vertical mesenterj', the aboral coelom being then undivided. The hydrocoel 

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



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



In figure 4 the oral calom occupies the whole space round the dorsal side of the entoderm. Tho 

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

 FiQ. 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 ia seen a 

 glandular cell. X750. 

 Figs. 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 ectodeim in 

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

 oral coelom are seen. In figure 7 the dorsal or aboral ca-lom 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, shovvmg 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 hydrocccl 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 shghtly 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 

 ia seen. 



PLATE XVIII. 



(All figures of Isomelra vifipara.) 

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



mass, consisting of phagocytes; the entodermal nuclei he 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 figures 



1 and 2. X165. 

 Fig. 3. Sagittal, longitudinal section of a fully formed larva with the vestibulum closed. X165. 

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



as figures 5 and G. 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 165. 

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



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



is indicated. X165. 

 Fio. 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. 



