MONOGRAPH OF THE EXISTING CRINOIDS. 487 



Between the body wall and the two layers of the fold there is seen in cross 

 section a roughly quadrangular space, traversed only by a few plasmatic processes. 



These folds persisted in the most developed larvre examined by Seeliger. 



Younger larvse, at the end of the second week, give the explanation of the 

 origin of this union. In these it can be easily seen that the common origin of 

 each pair of interradial tentacles is enveloiied at the base by a common ectodermal 

 sheath, from which the two points arise. The same condition is found at the 

 base of each group of radial tentacles, but here the common basal ectodermal 

 mantle is not enlarged, becoming instead eventually divided to the very roots 

 of the tentacles. 



Simultaneously with the spreading out of the ectoderm of the 15 radial 

 tentacles over an enlarged surface, there takes place within it marked histological 

 changes during which the original uniformity of the cells disappears. 



A cross section through a radial tentacle of an 11-day larva shows the nuclei 

 gathering into groups, the groups being separated by small intermediate zones, 

 in which nuclei are sparsely distributed or are entirely absent. 



In the regions where the nuclei are grouped, indicating the places of origin 

 of the papillie, the ectoderm is thickened and elevated, protruding in the form 

 of a peg, which appears hyaline and almost homogeneous and is composed of 

 the metamorphosed outer ends of the protoplasmic cell bodies. Cell borders can 

 not be determined either in the outer part or in the thickened basal section. 



The nuclei are often seen in process of division. Sometimes a small nucleus 

 can be seen in the distal cuticular tip of the structure. Whether this is natural, 

 or artificialy induced, has not been determined, but at all events in the fully 

 formed papilhe nuclei occur only in the basal part. 



The papillae, as becomes increasingly more evident in later stages, are arranged 

 in four somewhat irregular longitudinal rows. Those situated basally are less 

 advanced in development than the distal. 



In larvae four or five weeks old the papillae have reached the perfected form 

 and resemble in all essentials those of the ambulacral regions of the adult animals. 



The tentacles themselves have become rather long, very active organs, which 

 taper slightly toward the distal end where they are more densely covered with 

 papillae than at the base, so much so, indeed, that the bases of these are every- 

 where in mutual contact, and the epithelium therefore appears uniformly beset 

 with nuclei, without intermediate zones. Here the papillae are especially well 

 developed, measuring at their bases more than one-third of the circumference of 

 the tentacles, which is here considerably reduced. Since their arrangement in 

 longitudinal rows, though more or less disturbed, persists to the tip, they have 

 become considerably displaced by crowding. Thus on a cross section relatively few 

 are cut across, but farther basalward, toward the middle of the tentacle and be- 

 yond, the arrangement of the papillae in four longitudinal lines is clearly marked, 

 even though the four in each circumtentacular ring do not always stand at pre- 

 cisely the same height, and at the same time they become more isolated, with 

 increasingly extensive areas of thin clear ectoderm without nuclei between them. 

 142140— 21— Bull. 82 33 



