94 STUDIES IN THE DEVELOPMENT OF CRINOIDS. 



Figure 3 is below the stomach; the dorsal ccelom has disappeared, the space in the middle repre- 

 senting solely the oral coelom. Three glandular sacs are seen on either side. 

 Figure 1 is eighteen sections removed from figure 6 of the foregoing plate, and there are eighteen 

 sections between this and the following figure. Between figures 2 and 3 there are eleven 

 sections. X70. 

 FIGS. 4, 5. Posterior part of two sections from a sagittal, longitudinal series. 



Figure 4 shows the closed end of the pore canal; on the left side of the figure is seen a glandular 



sac in oblique section. 

 Figure 5, which is five sections removed towards the median line, shows the same glandular sac 



opening to the exterior (though not quite so distinctly as shown in the figure). X105. 

 FIGS. 6, 7. From a longitudinal, frontal series of sections. The figures represent only the anterior part, 

 showing the nervous system below the thickened ectoderm. At the lower end of the figures 

 the anterior end of the vestibulary invagination is indicated. 



Figure 6 is the more dorsal of the two figures. The thickening of the ectoderm in the anterior 

 end of figure 7 is due to the suctorial disk, the upper part of which is just touched by the 

 section. X 105. 

 FIG. 8. Part of the mesenchyme surrounding the stalk and the chambered organ; showing the yolk-granules, 



single or in globular masses. X290. 



FIG. 9. Part of the ectoderm, from the transitional region between the thick anterior and the thin posterior 

 part; showing the outer layer of small nuclei and the inner layer of larger nuclei, probably 

 belonging to the glandular cells. X290. 



FIG. 10. Part of the ectoderm in the vestibulary invagination. X780. 



FIG. 11. Part of the wall of a glandular sac. On the inner end of the cells are seen small masses of globules, 

 evidently resulting from the secretory function of the cells. Picrocarmine. X750. 



PLATE XXVII. 



(All the figures represent Floromctra scrratissima.) 



FIG. 1. Young Pentacrinoid. The anal plate has appeared; in two or three of the primary tentacles the first 



rudiment of the axillary was seen, but not in the anal radius. The specimen had sixteen 



stalk joints. X100. 

 FIG. 2. Another Pentacrinoid in the same stage of development. In the primary tentacle is seen a small 



calcareous body, the axillary. X100. 

 FIG. 3. Somewhat more advanced stage of the Pentacrinoid. The radialia have appeared (the small spicule 



to the right of the anal plate). The axillary is a rather large, fenestrated plate lying above 



the first sacculus. The first costal has not yet appeared. X100. 

 FIG. 4. The same stage, represented with the stalk in its whole length. X45. 

 FIG. 5. Later stage of the Pentacrinoid. The arms are distinct, consisting of a long costal and a large 



axillary. The radials are nearly joining and have separated the oralia from the basalia. X90. 

 FIG. 6. Part of a Pentacrinoid, slightly more advanced than that represented in figure 3. Between the 



axillary (which is slightly abnormally shaped) and the sacculus is seen a small spicule, the 



first rudiment of the costal. The radius figured is that to the left of the anal interradius. X 180. 

 FIG. 7. Part of the Pentacrinoid represented in figure 1, mounted in balsam after the dissolution of the 



skeleton. Showing the stone canal and pore canal, which has the appearance of being closed. 



The primary gonad is very indistinct. The anal opening has been formed. X290. 



PLATE XXVIII. 



(All the figures represent Thaumatometra nulrix.) 



FIG. 1. Newly attached Pentacrinoid, with the vestibulum still closed. X130. 



FIG. 2. A more advanced stage; the oral valves are open and the primary tentacles protruding. The radials 



have appeared. X130. 

 FIG. 3. Further stage of development. The radials have grown somewhat, and the costal I is appearing 



above them (in the figure distinguishable only to the left). The anal plate has appeared. 



The right side of this specimen has undergone some pressure, so that the oral plate to the right 



is lying nearly flat instead of in profile, as would be its natural position. X 105. 

 FIG. 4. Further advanced stage. The axillary has been formed and the arm-joints have begun to form. 



The incision in the oral plate to the right is a casual abnormality. X 100. 

 FIG. 5. Still further advanced stage; the arms are distinct, with about six joints. X60. 

 FIG. 6. Fully formed Pentacrinoid, with the first whorl of cirri in complete shape and the second whorl 



about to appear. The arms have been broken off. At the upper edge of the disk are seen the 



orals and the anal plate. The anal cone has been formed. X40. 



Fio. 7. Two lower stalk-joints of a Pentacrinoid, attached between two pinnule joints. X37. 

 FIG. 8. The stalk of a Pentacrinoid, which has detached itself. The stalk ia attached to the middle of a 



pinnule joint. X37. 



