92 STUDIES IN THE DEVELOPMENT OF CRINOIDS. 



PLATE XXI. 



(All figures except figure 7 represent Isometra viiripara.) 



FIG. 1. Young Pentacrinoid, decalcified. The vestibuluin not yet opened; the tentacles not yet protruding 

 into its lumen. X75. 



Fid. 2. Slightly older Pentacrinoid, decalcified. The tentacles have protruded into the vestibulum, which 

 has not yet opened; but there is a depression in the top of the wall and the oral valves ara 

 differentiating. The stone canal has lengthened considerably and the intestine has been differ- 

 entiated, but there is no anal opening formed as yet. X75. 



FIG. 3. Pentacrinoid, with beginning branching of the arms; decalcified. The opening of the pore canal is 

 seen. The little thickening in the mesentery passing below the stone canal is the primary 

 gonad. In the stomach folds are beginning to appear. Trabecules are seen in the coelomic 

 cavity. The chambered organ has widened considerably; from its middle the axial organ is 

 proceeding towards the stomach. The arms are not drawn in a very detailed way. X55. 



FIG. 4. Longitudinal section of a Pentacrinoid in a stage corresponding to that represented in figure 3. ' The 

 parietal canal is seen to open into the coslom. The outer opening of the pore canal may be 

 seen eight sections from the one figured. X 105. 



FIG. 5. Longitudinal section through a more advanced Pentacrinoid. The thickening at the upper side of 

 the hydrocoel is the place where the ring nerve (r. n.) is developing. X90. 



FIG. 6. Pentacrinoid of an advanced stage, decalcified. The anal opening has been formed. The primary 

 gonad is distinct; the axial gland is a distinct cord, continuing through the whole of the body 

 cavity. In the arm in the middle of the figure the primary tentacle is seen in the cleft. The 

 arms are otherwise not drawn in a more detailed way. At the base of the arms the interradial 

 tentacles are seen. X70. 



FIG. 7. Arm of a Pentacrinoid of Antedon bifida, showing the primary tentacle in the arm cleft. X65. 



PLATE XXII. 



(All figures of Isometra vivipara.) 



FIG. 1. The larva in its egg-membrane; seen from the ventral side; not specially treated. X75. 

 FIG. 2. A larva, stained in carmine and mounted in balsam, showing the band-like widenings from the 



anterior end of the vestibulary invagination. X75. 

 FIG. 3. Another larva, treated as the one represented in figure 2, but much less contracted, the anterior 



end with the suctorial disk standing out almost like a short proboscis. The widenings from 



the anterior end of the vestibulary invagination not so strong and band-like as in figure 2. X75. 

 FIGS. 4, 5. Two young larvse showing the first rudiments of the skeleton, viz, the five orals, the five basals, 



the terminal stem-plate, and a few stalk-joints. The specimen represented in figure 4 slightly 



older than that drawn in figure 5. X105. 



FIGS. 6, 7, 8. Full-grown larv with their skeleton, in ventral, dorsal, and side view. X105. 

 FIG. 9. Newly attached Pentacrinoid. X200. 

 Fio. 10. Later stage of Pentacrinoid; the radials have been formed. The orals have got a strong out-turned 



edge. X65. 

 FIG. 11. Further stage of development of the Pentacrinoid. Above the radials the costal and axillary 



have been formed. Spicules are seen in the tentacles. X 65. 

 FIG. 12. Later stage of development of the Pentacrinoid. The arms have already about 6 joints. The 



radials are joining one another, separating the orals from the basals. The anal plate is seen 



lying between the oral and the radial, both the latter having the corner joining it partially 



absorbed. X45. 

 (Figures 4, 5, 6, 7, and 8 have been drawn in a wrong position, with the anterior end downward.) 



PLATE XXIII. 



(All figures of Isometra vivipara.) 



FIG. 1. Newly attached Pentacrinoid. X45. 



FIG. 2. Slightly older stage; the stalk has lengthened and the orals have begun to form the out-turned lateral 

 edge. X45. 



FIG. 3. Further advanced stage. The radial plates have appeared. X50. 



FIG. 4. A slightly older Pentacrinoid, with two very young Pentacrinoids attached to its stalk. X20. 



FIG. 5. Further advanced stage. The costals and axillaries have been formed and the arms are beginning 

 to develop. X20. 



FIG. 6. Later stage of the Pentacrinoid. Several arm-joints have developed, and the stalk-joints are assum- 

 ing their final shape. X20. 



FIG. 7. The latest Pentacrinoid stage. Pinnute are beginning to develop, and the first whorl of cirri has 

 been formed. The little piece represented in figure 7a is the lowermost part of the stalk of the 

 same specimen. The terminal stem-plate was wanting. X20. 



