. Carnegie Institution of Washington publication. 22 STUDIES IN THE DEVELOPMENT OF CRINOIDS. lated, their surface looking coarsely thorny when seen in profile. Upon the whole this Pentacrinoid is by no means such a beautiful, delicate-looking object as are several other Pentacrinoids—for instance, that of Hathrometra prolixa. The oralia are but very slightly concave, as seen in figure 9, repre- senting the calyx seen from above. The stalk-joints now are 13 in number, 3 new ones having formed at the upper end. These upper joints are con- spicuously thickened in the middle, a feature which gradu
. Carnegie Institution of Washington publication. 22 STUDIES IN THE DEVELOPMENT OF CRINOIDS. lated, their surface looking coarsely thorny when seen in profile. Upon the whole this Pentacrinoid is by no means such a beautiful, delicate-looking object as are several other Pentacrinoids—for instance, that of Hathrometra prolixa. The oralia are but very slightly concave, as seen in figure 9, repre- senting the calyx seen from above. The stalk-joints now are 13 in number, 3 new ones having formed at the upper end. These upper joints are con- spicuously thickened in the middle, a feature which gradually disappears on th,e lower joints; from the sev- enth to eighth there is no thickening in the middle of the joints, which are now simply cylindrical; a dark line across the middle of the joint still indicates the ori- ginally formed plate, from which the joint develops by means of vertically growing processes, which unite by cross-beams, as described by Seeliger in Antedon (pages 325, 326). The final shape of the stalk-joints, as well as their definite number in the full-grown Pentacrinoid, remains, of course, unknown. The terminal stem-plate is an irregular disk with some short, rounded prominences (plate x, figure 10). The dark plate at the upper end of the stalk (plate x, figure 8) represents the infrabasalia, now consider- ably thickened, and destined to form, together with the upper stalk-joint (not the upper joint seen in this figure), the centrodorsale. A discussion of the infrabasalia will follow in the general part. While no later stage in the development of the Pentacrinoid was obtained, a young specimen, which was found by Dr. H. L. Clark, and which he most kindly gave me, gives some valuable information respecting the further development. The specimen had an arm-length of 10 mm., the diameter of disk being 2 mm. There are 10 pinnulse to each arm and 13 cirri. It would appear from this specimen that the order of appearance of the oral pinnules is the same as tha
Size: 1525px × 1639px
Photo credit: © Book Worm / Alamy / Afripics
License: Licensed
Model Released: No
Keywords: ., bookauthorcarnegie, bookcentury1900, bookdecade1900, bookyear1902
