MONOGRAPH OF THK EXISTING CRINOIDS. 



453 



Only in the most anterior portion of the larva does the membrane mentioned 

 above separate the nervous elements sharply from the body cavity; farther 

 posteriorly it becomes less and less prominent during the later larval existence, 

 while at the same time connective tissue fibers appear. 



In the center of the nervous apparatus directly under the apical pit there 

 lie on the fiber laj'er very numerous irregular ganglion cells arranged in several 

 layers. Laterally and farther posteriorly they become progressively less and 

 less common, and over the finer fiber strands none occur. 



Prominent on account of their size and constant in their course are two 

 nerve cords which arise from the central fiber layer under the attachment pit and 

 run right and left along the ventral side of the larva in the angles formed through 

 the vestibular invagination. In cross section they appear triangular, with the 

 smaller side toward the body cavity and the two longer toward the inner side 

 of the ectoderm, and the lateral wall of the vestibule. Posteriorly both become 

 progressively finer; they probably reach the fourth ciliated band. 



In the anterior portion isolated ganglion cells lie on the side toward the body 

 wall, but in the posterior portion these seem not to occur, and here only detached 

 ectoderm cells are found which have only partially withdrawn from the ectodermal 

 layer. These latter consequently resemble those in the apical pit at a younger 

 stage which are in process of moving to a subepithelial position as a preliminary 

 to transformation into ganglion cells. But in the posterior region of the two 

 ventral nerve cords the cells do not complete the transformation into ganglion 

 cells, since before sufficient time has elapsed for this the degeneration of the 

 whole nervous apparatus supervenes. 



During the later embryonal period the cells which remain in the epithelial 

 layer of the apical pit have become markedly altered, and, like the cells in the 

 ciliated bands, have greatly increased in number. They are now slender wandlike 

 cells, which possess a deeply stained usually elongate nucleus situated near their 

 inner end. In the central portion, where they are longest, they are from 0.025 

 mm. to 0.030 mm. in length, decreasing thence toward the border so that their 

 deeply stained nuclei in longitudinal sections through the region form an arc- 

 shaped band with the concavity toward the exterior. On their outer ends they 

 carry the long cilia which are deeply embedded in the cuticuiar cell ends, causing 

 the latter to appear longitudinally striped. Just under this longitudinally striped 

 zone there is a narrow band of deeply stained cytoplasm. 



On the basis of differences in the inner ends two sorts of cells can be differ- 

 entiated in the apical pit. One type has a clublike swollen or truncated end; 

 these are shorter than the others and lie irregidarly distributed among them; 

 their nuclei lie quite near their inner ends. These appear to be supporting cells. 

 The second type tapers off into an exceptionally fine and long process which 

 extends between the ganglion cells in the fibrous layer. These are to be inter- 

 preted as sense cells having to do especially with taste, since from the reactions 

 of the larvae a certain amount of the ability to taste is to be inferred. 



