MONOGRAPH OF THE EXISTING CKINOIDS. 485 



arms and pinnules, everywhere preserving in all essentials its original histological 

 characteristics. 



In some of his oldest larvae Seeliger saw the cylindrical epithelium of the 

 disk extend unmodified to the water vascular ring without passing through a 

 broad intermediate zone, by which the gradual transformation to the pavement 

 epithelium is brought about, as is commonly the case. He assumes that this results 

 from the extension of the epithelium through an increase in the plasma content 

 of the cells, whereby they are changed from the strongly flattened to the prismatic 

 type resembling the form of the ancestor cells at the time the vestibule was still 

 closed. 



There is no connection whatever between the nervous system of the free- 

 swimming larva and the nerve ring of the stalked larva which develops into the 

 ectodermal (ventral) nervous system of the adult. The latter first appears in 

 the fourth week, a long tune after the rupture of the vestibule, as a derivative from 

 the ectoderm of the original vestibular invagination on the region where the 

 epithelium of the disk passes over into that of the tentacles. In longitudinal 

 sections this region is very early noticeable on account of the relatively high 

 ectoderm cells which show closely crowded nuclei, contrasting strongly with the 

 pavement epithelum on either side. 



The nerve ring lies close down on the hydroccele ring which, as will be 

 noticed subsequently, is remarkable for the extremely strong ring muscular 

 fibrillse. When these contract they press upon the nerve ring, and its borders 

 stand out less sharply. When the calyx is opened out the two structures are 

 separated by an intermediate space. 



At the first appearance of the nervous system the ectoderm of the region 

 where it is found is composed of a single layer of cells which either have no definite 

 borders at all, or show them only in their outer ends. Inwardly, toward the ring 

 muscle of the hydroccele, the cell ends pass over into a substance which retains 

 the stain rather more tenaciously than the outer part and appears fibrillar, with 

 coarse ring fibrillse running parallel to the surface which appear in cross section 

 as coarse grains. The nuclei, rich in chromatin, are very prominent; they lie 

 in the outer ends of the cells, though not all at the same level. Here and there 

 one may be seen undergoing division. There is no doubt that the ring fibrillse, 

 which are nerve fibrillse, as well as the intermediate substance in which they are 

 embedded, have been formed from the basal part of the ectoderm cells. The 

 appearance of the nerve ring is not simultaneous throughout its entire extent; 

 it seems to form on several places simultaneously, gradually growing into a closed 

 ring. But there is a possibility that the apparently nerveless intermediate zone 

 may possess a very fine ring nerve not demonstrable on preserved material. 



In larvae five weeks old the development of the nerve ring is somewhat 

 further advanced. Over the now larger nerve fibrillar layer the ectodermal 

 epithelium is now no longer one layered, for a number of cells have detached 

 themselves and assumed a subepithelial position, in part lying with their outer 

 ends between the bases of the epithelial cells, in part already entirely isolated from 



