86 STUDIES IN THE DEVELOPMENT OF CRINOIDS. 



Figure 7 shows the axial organ and the outer end of the rectum. In the vestibulum are seen the 

 sections of 3 of the primary tentacles. 



Figure 8, which is separated from 7 by 3 sections, shows the stone canal (st. c.), which is in connec- 

 tion with the hydroccel ring two sections from this. 



In figure 9, which follows immediately upon 8 in the series, the stone canal is seen to open into 

 the parietal canal. The latter continues through 3 more sections and then the pore canal 

 appears. In the left mesentery is seen an accumulation of cells, representing the primary 

 gonad (pr. g.). 



Figures 7 and 8, X290; figure 9, X375. The small spaces seen in a row in the epidermis are from 

 the oral plates; in figure 9 also from one of the basal plates. 



PLATE VII. 



(All figures of Tropiomelra carinata. All X290.) 



Fid. 1. From a transverse section of an embryo in the Pentacrinoid stage, showing the separate origin 

 from the hydroccel ring of the oral tentacles. The figure is part of the section following 

 directly after that represented in plate vm, figure 5, viz, the radius marked with an asterisk (*). 



FIQ. 2. Section through the stalk of a pentacrinoid (the same as plate vm, figures 4 to 8), shows the 

 chambered organ. 



FIQ. 3. Longitudinal section through an embryo 8 days old, just attached. The vestibulum is closed 

 normally and the mouth opening is about to form. The calomic spaces are not very clear 

 in this series. 



FIQ. 4. Longitudinal lateral section through an embryo 4 days old. The vestibulum is not normally closed 

 and the embryo is pipe-shaped. The lumen of the entoderm is very spacious in this specimen, 

 and no cells have wandered into it. 



Fio. 5. Longitudinal median section through another embryo 4 days old, pipe-shaped, with the vestibulum 

 not normally closed. The relations of the ccelomic spaces not quite clear in this series; the 

 oral ccelom apparently in open connection with the aboral coelom. 



FIGS. 6, 7. Longitudinal sections through an embryo 8 days old, normally attached and with the vestibulum 

 normally closed. In figure 6 the stalk is omitted. The left side of the figures is the anal side. 

 Figure 6 shows the chambered organ and the rudiment of the axial organ in the vertical 

 mesentery. Figure 7, which is slightly lateral, shows that the esophagus and mouth have 

 opened into the vestibulum. On the left side of the figure one of the primary tentacles has 

 been cut longitudinally. 



FIG. 8. Longitudinal section of an abnormal, pipe-shaped embryo, 8 days old, with the vestibulum incom- 

 pletely closed. The axial organ more developed than in the specimen represented in figure 6. 

 In the incomplete vestibulum is seen a primary tentacle cut; the mouth-opening is about 

 to develop. 



PLATE VIII. 



(All figures of Tropiometra carinata. All X290). 



FIQ. 1. Median, longitudinal section through a normal Pentacrinoid (attached to the surface film), the 

 vestibulum still closed. The pore canal is seen close to the surface at the left side; it is present 

 in two sections more, but has no opening to the exterior; the skin forms a little elevation 

 over it. A thick membrane is formed over the basal surface of the stalk. The chambered 

 organ is seen to continue almost to the basal surface of the stalk. (The part of the figure 

 representing the stalk combined from two to three sections.) 



FIGS. 2, 3. From the same series as figure 1. In figure 2, which is separated from 1 by two sections, the 

 pore canal is seen joining the parietal canal; in figure 3 the parietal canal opens into the 

 oral co?lom; the stone canal is seen just above the parietal canal. 



FIGS. 4, 5, 6, 7, 8. From a series of horizontal sections through a normal Pentacrinoid in the same stage as 

 that figured in figures 1 to 3-. 



Figure 4 shows the tentacles, 3 in each radius, in the vestibulary cavity. They are separated from 

 one another by fine but distinct lines, which seem to indicate the presence of a slimy substance 

 in the vestibulary cavity. 



Figure 5, which is separated by four sections from figure 4, is in the level of the origin of the tentacles 

 from the hydrocoel ring. Here the hydropore is seen to open to the exterior. Besides the 3 

 radial tentacles, 2 interradial tentacles are also developed (cf. plate VH, figure 1, which is taken 

 from this series). 



Figure 6, which is separated from figure 5 by two sections, is in the level of the hydroco?! ring, 

 which is seen nearly complete in the section. Numerous fine trabecules are seen within the 

 ring. The narrowing in the anal (upper) interradius is the point of closure of the ring. 



Figure 7, separated from figure C by four sections, shows the stone canal opening into the parietal 

 canal ; this section goes through the esophagus. 



Figure 8, separated from figure 7 by two sections, passes through the rectum, which is seen to have 

 not yet opened to (he exterior. 



