PART 5 A MONOGRAPH OF THE EXISTING CRINOIDS 9 



In these larvae ciliated bands are wholly absent, and neither is there a general 

 ciliation of the surface. Cilia are distinctly to be seen only in the suctorial disk and in 

 the vestibular invagination. 



The ectoderm is much thickened over the whole of the anterior end, gradually 

 thinning out posteriorly. The extent of the thickened portion is variable; in some 

 cases it passes below the hydrocoele, and in others it is confined to the part beyond 

 the stomach and the hydrocoele. In the thickened portion of the ectoderm the histo- 

 logical details are difficult to make out. It is a mass of nuclei intermingled with 

 glandular cells. In the region where the transition to the thinner portion takes place 

 there can be discerned an outer layer of small closely aggregated nuclei and an inner 

 layer of larger nuclei, between which is a mass of irregular threadlike structures. Mor- 

 tensen says it can scarcely be doubted that this corresponds to the glandular cells 

 found in the skin of other crinoid larvae, and it would appear that the inner larger 

 nuclei belong to the glandular cells. In the thin part of the ectoderm only a few small 

 nuclei lie close to the surface, and glandular cells are seen only here and there. 



The ectoderm of the vestibular invagination differs from the external ectoderm 

 only in being much thicker. The floor of the vestibule is richly provided with glandular 

 cells from a point a Uttle below its anterior end to about where the hydrocoele begins. 

 There is no such shifting of the glandular and nuclear portions as there is in Isometra 

 vivipara. The fuiTow is very distinctly ciliated. The cilia perforate the cuticle. 

 Within the cuticle a darker line is visible, not a series of fine grains as was observed 

 in Isometra. 



The nervous system is remarkably developed for a larva devoid of both the power 

 and opportunity for movement and therefore, seemingly, with no special need for it. 

 It forms a conspicuous layer beneath the epidermis at the anterior end, reaching almost 

 as far as the posterior end of the vestibular invagination. It is more restricted ven- 

 trally and is only visible beneath the suctorial disk, whence it is continued posteriorly 

 as a distinct nerve running along either side of the vestibular invagination. 



The endoderm forms a capacious sac with thin walls in which the nuclei are arranged 

 in a layer of somewhat varying thickness, in many places even in a single layer. The 

 lumen of the sac is entirely empty. 



The hydrocoele forms a half ring Ijong in the usual place a little to the left of the 

 vestibiilar invagination. The primary tentacles are just beginning to form, as is indi- 

 cated by thickenings of its epithelium. A slight prolongation toward the parietal canal 

 probably indicates the beginnmgs of the stone canal. The pore canal is closed. The 

 parietal canal has a long and narrow anterior prolongation. 



The coelomic vesicles have assumed their normal position, the left at the posterior 

 (the future oral) end, the right at the dorsal side. The former extends upward on 

 either side of the hydrocoele. 



The chambered organ has been formed, and may be followed to near the anterior 

 end. 



Mortensen notes that while there is nothing unusual in the relations of the endo- 

 derm, the hj^drocoele, and the parietal canal, a very unusual feature is apparently 

 connected with the oral coelome. In the posterior end of the embryo are a number of 



