638 BULLETIN 82, UNITED STATES NATIONAL MUSEUM VOLUME 1 



stage. As a result of this, the yolk spherules wander into the lumen, where they are 

 evidently being absorbed. The nuclei, which in the stage preceding were lying fairly 

 regularly along the inner border of the entoderm, are now spread over the whole 

 entodermal mass without any apparent order, evidently as a result of having undergone 

 division. 



The hydrocoele is now assuming the shape of a horseshoe, open downward; its 

 walls consist of high cyclindrical cells which form a regular columnar epithelium. The 

 parietal canal has developed into a large sac with a lining of flattened endothelial cells. 

 It has a large anterior prolongation reaching to just below the suctorial disk, where it 

 may be a little widened. At its lower end it is constricted into a long narrow pore 

 canal, which may have an external opening, the hydropore, situated between the third 

 and fourth ciliated bands. In some of the specimens studied by Mortensen there was 

 no hydropore; he ascribes this to variability in the time of appearance of this structure. 



The enterocoelic vesicles have now reached their definitive positions, the left 

 (the future oral coelome) at the posterior end, the right (the future aboral coelome) 

 at the dorsal and anterior side of the entoderm, forming a mesentery at the point of 

 union. From the right enterocoelic vesicle or aboral coelome are developing prolonga- 

 tions into the anterior part of the embryo which are the rudiments of the chambered 

 organ. The epithelial lining of the enterocoele vesicles has for the most part assumed 

 an endothelial character. 



In the mesenchyme there are often found globular masses of yolk spherules of 

 different sizes, sometimes very large. More rarely such yolk globules are also found 

 in the ectoderm. In the entoderm the yolk spherules are usually not united into 

 definitely delimited globules. 



The fully formed larva still within the egg membrane is from 0.5 to 0.6 mm. in 

 length about the size of the corresponding larva of Antedon adriatica, and about twice 

 the size of the larvae of Tropiometra carinata or of Antedon serrata. In shape it is 

 markedly different from the other known crinoid larvae. It is flattened on the ventral 

 side and has a distinct constriction in the middle of the body between the oral and 

 basal circlets. The vestibular invagination is in the form of a narrow slit. Its depres- 

 sion is not sharply delimited from that of the suctorial disk. The latter is very distinct 

 and often has thickened edges which causes the anterior end of the animal to appear 

 prominent and almost snoutlike. The ciliated bands are very well developed, 4 in 

 number with traces of an anterior band which is very indistinct and only visible on the 

 dorsal side. The second (pre vestibular) band may be interrupted in the ventral median 

 line between the suctorial disk and the vestibule. The next band is forced far posteriorly 

 by the vestibule, reaching almost to that following which also is slightly curved posteri- 

 orly in the midventral line. The posterior band may curve slightly anteriorly in the 

 midventral line. At the anterior end of the vestibule a band may proceed laterally, 

 like an additional rudimentary ciliated band. This, however, is not constantly present, 

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

 end of the vestibule. 



The ectoderm is only very exceptionally distinct from the mesoderm; usually no 

 delimitation can be made out between them. It is evidently at this stage that the 

 dissolution of the ectoderm as a separate layer takes place. The nuclei of the cells 

 of the ciliated bands are arranged in conspicuous groups, though rarely showing any 

 regular arrangement within these groups. The glandular cells have become enormously 



