COMPSOMETRA SERRATA. 25 



Tropiometra, I find the nuclei of the ectoderm distinctly smaller than those 

 of the rest of the embryo. 



The next stage represented is the fully formed larva (plate xn, figures 

 1 and 2). The length is only about 0.3 mm.; the shape is elongate, the 

 hind end slightly pointed. The vestibulary invagination is broadly oval; the 

 suctorial disk is distinct, as is also sometimes the apical pit. There are 5 

 ciliated bands, but the anterior one is rudimentary and seen only on the 

 dorsal side. The third band appears to be interrupted by the vestibulary 

 invagination; it is only slightly bent downwards. I have been unable to trace 

 the continuation of it inside the edge of the vestibulary invagination. The 

 fourth band is hardly bent downwards on the ventral side, and neither is 

 the band at the anterior end of the vestibulary invagination bent upwards. 



The interior organization of the larva shows considerable progress from 

 the former stage (plate xi, figures 5 to 10). The hydrococl has formed the 

 5 lobes representing the primary tentacles; the parietal canal, which is large, 

 with a conspicuous anterior prolongation (figures 8 and 9), communicates 

 with the exterior through the pore canal which opens between the third and 

 fourth vibratile band (figure 10). The outer end of the pore canal is some- 

 what widened. The shape and arrangement of the two enterocoel vesicles 

 appears from a comparison of the transverse sections (figures 5 to 7) with the 

 longitudinal sections (figures 8 to 10). The chambered organ is seen to con- 

 tinue towards the anterior end (figures 5 and 8). The stomach may be a 

 large sac (figure 9) or flattened, so that the lumen can hardly be discerned 

 (figure 7). The nervous system is very well developed (figures 8 and 10) 

 and as usual it continues as a distinct nerve along each border of the vestibu- 

 lary invagination (figure 5). In the ectoderm glandular cells are numerous 

 in the anterior end, in the region of the apical pit. Also in the vestibulary 

 invagination they are fairly well developed, though very much less so than 

 in Tropiometra. In the rest of the ectoderm glandular cells are hardly present 

 at all. The nuclei of the ciliated bands are beautifully arranged in arcs, the 

 surface showing a corresponding concavity, more or less distinct. 



In plate xn, figure 5, is represented a young Pentacrinoid, decalcified. 

 The pore canal is remarkably short and its outer opening apparently closed. 

 This seems to be in conformity with Russo's observation that the hydro- 

 pore of the Antedon larva disappears, and a new, secondary pore develops 

 in its place. 16 I must, however, say that I do not venture to assert that this 

 is also the case in Compsometra, the histological character of the material, pre- 

 served in alcohol, not being sufficiently clear to reveal such minute details with 

 all desirable distinctness. The pore really appears to be closed, but it may 

 perhaps be due to contraction. In any case this deserves closer investigation. 



The stone canal appears to have the shape shown in the figure quoted, 

 but it could not be made out with certainty. The inner half of the parietal 



' Russo (op. cit., pp. 47, 48). 





