12 STUDIES IN THE DEVELOPMENT OF CRINOIDS. 



ultimately results in the complete intermingling of the ectoderm cells in the 

 mesenchyme, so that in the later stages of development there is no separate 

 ectoderm layer. The details of this process could not be followed in Tropio- 

 metra, but since the first stage of the process and also the end-result are the 

 same in Tropiometra as in Antedon, there is evidently no reason to doubt 

 that the whole of the process is identical with that obtaining in Antedon, as 

 described by Seeliger. An indication of this intercellular substance is also 

 seen in plate iv, figure 1, at the posterior end, as well as in plate iv, figure 3, 

 these figures being likewise from embryos 25 hours old. 



In plate iv, figure 13, from an embryo 40 hours old, is seen a depression 

 just in front of the vestibulary invagination, evidently representing the 

 suctorial disk, by means of which the larva attaches itself. It is very indis- 

 tinctly seen, except in sections. The anterior band passes uninterrupted 

 between the disk and the invagination, as is also the case in Antedon. 



The ciliated bands generally appear in sections as slightly concave struc- 

 tures (plate iv, figure 13); sometimes, however, they are distinctly convex 

 (plate v, figure 3), while at other times they appear to be nearly flat. The 

 differences are evidently due to preservation. 



The nervous system, first seen by Bury in the Antedon larva and described 

 in a detailed way by Seeliger, appears to exist also in the Tropiometra larva 

 in at least the same degree of differentiation, or probably even somewhat 

 more strongly developed. In the anterior end, below the apical tuft, is 

 seen a conspicuous layer of an exceedingly fine fibrillary substance, which 

 continues more or less distinctly downwards, below the vibratile bands 

 (plate iv, figures 13 and 14; plate v, figures 3, 4, and 5). The nerve passing 

 along the side of the vestibulary invagination may also be observed (plate v, 

 figure 6, distinct on the right side of the figure only), but not so distinctly 

 as appears to be the case in Antedon. Unfortunately the preservation is 

 not so good as to permit the determination of its nervous character from 

 the histological structure, but from the complete analogy of its position with 

 the nervous system of the Antedon larva it would appear beyond doubt that 

 this structure in the Tropiometra larva represents the nervous system. 



The codom. In embryos 16 hours old the right and left ccelomic vesicles 

 have separated completely and are about to assume their final position, the 

 left at the posterior end of the embryo, the right along the dorsal side, 

 covering the entoderm. The series of transverse sections of an embryo 20 

 hours old (plate in, figures 8 to 11), of an embryo 25 hours old (plate iv, 

 figures 4 to 12), and of an embryo 40 hours old (plate v, figures 7 to 10), com- 

 bined with the longitudinal sections of corresponding stages represented in 

 the same plates, will show the shape and extent of the two ccelomic vesicles, 

 making a detailed description superfluous. It is seen that the right vesicle 

 widens gradually, so that at the age of 40 hours it occupies the whole dorsal 

 side in the posterior half of the embryo. The epithelium of the vesicles 



