ILLUSTRATIONS. 91 



PLATE XIX. 



(All figures of Isomclra vimparn.) 



Fias. 1, 2, 3, 4. From a scries of longitudinal sections of a newly attached Pentacrinoid, corresponding to 

 plate xviit, finures 1 and 2, showing the interrelations of stone canal, parietal canal, and pore 

 Ciinal. In figure 1 is seen the origin of tlie stone canal from the hydroctjcl ring; in figure 2 the 

 hydroca'l has almost disappeared, the stone canal originating from the end of the still horse- 

 shoe-shaped hydroco'l. Figure 3 shows the end of the stone canal, which docs not yet open 

 into the ])ari(Ual canal. The space appearing above the stone canal is the oral ca'loni. In 

 figure 4 the stone canal has disai)i)eared and only the parietal canal and pore canal are seen ; 

 the latter can be traced through four more sections, but there is no outer opening. Between 

 the parietal canal antl the entoderm some nuclei arc seen which may indicate the primary 

 gonad. There are four sections between figures 1 and 2, three sections between figures 2 

 anti 3, and one section between figures 3 and 4. The entoderm is filled with a granular mass, as 

 in plate xviii, figures 1 and 2. X290. 



Fig. 5. Transverse section of a Pentacrinoid, containing a half-digested larva in the stomach. The section 

 is somewhat oblique, which accounts for the fact that only part of the hydrocoel is seen, the 

 two ends of which are represented, separated by a narrow wall. The oval space outside the 

 hydrocoel is the pore canal. The thickening of the entoderm in the anal interradius is noticeable. 

 F'igures 1 and 2 of plate xx represent two more anterior sections of the same specimen, the 

 latter separated from the section here represented by 25 sections, X165. 



Figs. 6, 7, 8, 9, 10. Series of figures from the same specimen showing the shape of the entodermal thickening 

 in the anal interradius. 

 In figures 6 and 7 the stone canal and parietal canal are seen; figure 8 goes through the rectum, the 



anal opening being indistinct on account of the pressure caused by the devoured larva. 

 In figure 9 the entodermal thickening has closed into a canal which is seen in the three consecu- 

 tive sections. 

 Figure 10 is some distance below the thickening; only a small ridge is seen here. 

 There are five sections between figures 5 and 6; two sections between figures 6 and 7; seventeen 

 sections between figures 7 and 8; nine sections between figures 8 and 9, and twenty-five sections 

 between figures 9 and 10. X165. 



Fia. 11. Part of a section of a young larva with beginning formation of the vestibulary invagination, show- 

 ing the distribution of the yolk spherules in all parts of the larval tissues: ectoderm, hydrocoel, 

 entoderm, and calom. Stained with hematoxylin and eosin. X535. 



PLATE XX. 



(All figures of Isomclra vivipara.) 

 Fios. 1, 2. Transverse sections of a Pentacrinoid; other sections from the same series figured in plate xix> 

 figures 5 to 10. 



Figure 1 is through the upper part, above the mouth. The five concave parts are the oral valves, 

 between which the arms protrude. 



Figure 2 is further down, at the base of the arms. The mouth has been protruding like a funnel 



in this specimen and thus appears as a ring in the section. The direction is somewhat oblique, 



which accounts for the fact that sections of tentacles are seen in part only of the figure. X 165. 



Figs. 3, 4, 5, 6. Transverse sections of a Pentacrinoid of a stage corresponding to that represented in plate 



XXI, figure 6. 



Figures 3 and 4 show the hydrocoel ring with its trabecula;. The fact that the hydrocoel is not seen 

 in the interradius turned up in the figure must be due to some accidental contraction and to a 

 slight obhquity of the section, not to a normal sinuosity of the ring. In figure 3 is seen the 

 opening of the pore canal and at * the narrow wall still separating the two ends of the hydrocoel. 



Figure 4 shows the parietal canal and the stone canal, partly horizontal and partly in transverse 

 section. The ccelom has sent prolongations into the arms. A thickening is seen in the eso- 

 phageal wall in the anal interradius. 



Figure 5 shows the anal opening. 



Figure 6 is from the middle part of the body, showing the intestine in longitudinal section. In both 

 figures the axial organ is seen between the stomach and intestine, as a hollow cord. Noticeable 

 is the concavity corresponding to each arm in which the ccelomic epithelium is much more 

 closely nucleated than in the rest of the ccelom. This concavity continues into the arms, 

 forming the arm-ccclom. 



There are eight sections between figures 3 and 4, thirty-five sections between figures 4 and 5, and 

 twenty-eight sections between figures 5 and 6. Figure 3 is to a slight extent combined from 

 two sections, the opening of the pore canal being distinct only in the section following upon 

 the one that has been drawn. All four sections X75. 

 Fios. 7, 8. Part of sections lower down in the same series, showing the chambered organ (figure 8, somewhat 

 obhque) and its continuation into the stalk (figure 7). X290. 



