PART 5 A MONOGRAPH OF THE EXISTING CRINOIDS 169 



In the youngest stage observed by Dr. Mortensen the hydrocoelc has occupied its 

 place on the ventral side and the parietal canal is about to separate from it. The two 

 enterocoele vesicles have separated complete^ from one another and occupied theu" 

 usual position, the left at the posterior end, the right at the dorsal side of the endoderm. 

 Unfortunately this stage is too far advanced to show whether the endoderm sends out 

 posterior prolongations to embrace the central part of the enterocoele vesicle as it does 

 in other species of Antedon, but not in Tropiometra. The walls of the anterior end of the 

 right enterocoele are thicker than those of the rest of the vesicle, and it would appear 

 that this has something to do with the formation of the chambered organ. Dr. Morten- 

 sen was at first inclined to believe that in this case the parietal canal might be forming 

 from the right enterocoele vesicle instead of from the hydrocoele, but the question 

 was settled by the oblique sections which showed that the formation of the parietal 

 canal is in conformity with what obtains in other species of Antedon and in Tropiometra. 



The vestibulary invagination and the suctorial disk have begun to form, and the 

 cUiated bands have begun to develop as is evident from the arrangement of the cells 

 in the ectoderm. There are no glandular cells in the ectoderm. The apical pit Dr. 

 Mortensen did not find distinct. As in Tropiometra the nuclei of the ectoderm cells 

 are distinctly smaller than those of the rest of the embryo. 



The next stage represented is the fully formed larva. The length is only about 

 0.3 mm.; the shape is elongate, with the posterior end slightly pointed. The vestibu- 

 lary invagination is broadly oval. The suctorial disk is distinct, as is sometimes also 

 the apical pit. There are 5 ciliated bands, but the anterior is rudimentary and is 

 visible only on the dorsal side. The thu-d band appears to be interrupted by the 

 vestibular invagination; it is only slightly bent downward. Dr. Mortensen was un- 

 able to trace its continuation within the border of the vestibular invagination. The 

 fourth band is scarcely bent downward on the ventral side, and neither is the band at 

 the anterior end of the vestibular invagination bent upward. 



The internal organization of the larva shows considerable progress. The hydro- 

 coele has given off the 5 lobes representing the 5 primary tentacles; the parietal canal, 

 which is large ^"ith a conspicuous anterior prolongation, communicates with the ex- 

 terior through the pore canal which opens between the third and fourth ciliated bands. 

 The outer end of the pore canal is somewhat broadened. The shape and arrangement of 

 the two enterocoele vesicles appears from a comparison of the transverse with the 

 longitudinal sections. The chambered organ is seen to continue toward the anterior 

 end. The stomach may be either a large sac or flattened, so that the lumen can scarcely 

 be discerned. The nervous system is very well developed, and as usual it continues 

 as a distinct nerve along each border of the vestibular invagination. In the ectoderm, 

 glandular cells are numerous in the anterior end, in the region of the apical pit; they 

 are also fairly well developed in the vestibular invagination, although very nmch less 

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

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

 the surface showing a corresponding concavit}', more or less distinct. 



Dr. Mortensen figures a young pentacrinoid, decalcified. In it the pore canal is 

 remarkably short and its outer opening apparently closed. He remarks that this 

 seems to be in conformity with Russo's observation that the hydropore of the larva of 

 Antedon mediterranea disappears and a new secondary pore develops in its place. He 

 does not venture definitely to assert that this is the case in A. serrata, since the 



