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 that found in Compsometra serrata (see page 28). The ten- 

 tacles are studded with small, simple spicules, generally lying in a series 

 along one side of the tentacle. 



FIG. 3. Oral region of a specimen of Tropiometra, 2 mm. 

 in diameter, X175. a, ambulacral furrow; h, primary hydro- 

 pore; ir. t, interradial tentacles; m, mouth; o, oralia; s, sacculi; 

 t, tentacles. 



