38 STUDIES IN THE DEVELOPMENT OF CRINOIDS. 



nation is not sharply limited from that of the suctorial disk (plate xxn, 

 figure 8). The vibratile bands are very well developed. There are four of 

 them. Traces of an anterior band may be seen, but it is very indistinct 

 and is seen only on the dorsal side. The second band, the prevestibulary 

 band, as it may be termed, may be interrupted in the ventral middle line 

 between the suctorial disk and the vestibulum (plate xxn, figure 3). Of the 

 postvestibulary bands, the upper is bent strongly downwards by the vestib- 

 ulum, reaching almost the next band, which is also bent slightly downwards 

 in the middle line (plate xxn, figures 1 to 3). The posterior band may 

 have a slight bending upwards in the ventral midline. A very remarkable 

 feature in this larva is found at the anterior end of the vestibulum, from 

 which a band may proceed laterally, like an extra, rudimentary ciliated 

 band (plate xxn, figure 2) ; it is, however, not constantly developed, and 

 there may be only a simple widening of the thickened epithelium at the 

 upper end of the vestibulum (plate xxn, figure 3). 



The size of the fully formed larva is 0.5 to 0.6 mm. in length, being thus 

 twice the size of the larva of Tropiometra and Compsometra, while the larva of 

 Antedon also reaches the size of about 0.5 mm. (Seeliger, p. 231). It is still 

 lying within the egg-membrane, as shown in the figures on plate xxn. 



The ectoderm (plates xvi to xvn) is only more exceptionally distinctly 

 limited from the mesoderm (as in the series, plate xvn, figures 9 to 12); 

 generally no limit can be made out between ectoderm and mesoderm, and 

 it is evidently in this stage that the dissolution of the ectoderm as a separate 

 layer takes place (see above, p. 11-12, for Tropiometra). The nuclei of the 

 vibratile bands are arranged in conspicuous groups, although rarely showing 

 any regular arrangement within these groups (plate xvi, figures 6 and 7; plate 

 xvn, figure 8). The glandular cells have become enormously developed, 

 especially on the ventral side and along the vestibulum (plate xvi, figures 

 2 to 5). In the vestibulum a regular arrangement of the glandular cells is 

 apparent. At the anterior end of the vestibulum the glandular cells occupy 

 the bottom of the furrow, while the sides are occupied by simple cells, seen 

 in the sections as very conspicuous masses of nuclei. Proceeding downwards, 

 the nuclear masses gradually pass down into the furrow, narrowing the gland- 

 ular space in the bottom, until it disappears completely. Then the nuclear 

 masses occupy the bottom; in the same time new glandular masses appear 

 along the sides of the furrow, these in their turn again going deeper down in 

 the furrow, narrowing the nuclear mass to a narrow space in the middle of 

 the furrow, while again new nuclear masses appear along the sides, these 

 latter, however, remaining less conspicuous. This peculiar arrangement is 

 seen in the series of transverse sections represented in plate xvn, figures 

 1 to 4, 6, 7, and 9 to 12, as also in the longitudinal sections plate xvn, figure 

 8; plate xvi, figures 3 and 9; and plate xvm, figures 3 and 8. In the sec- 

 tions (stained with hematoxylin-eosin) it is seen very beautifully, the gland- 



