i82 Papers from the Marine Biological Laboratory at Tortugas. 



the epithelium near the left side of the figure is a nematoblast from which 

 the larger type of nematocysts is formed. 



Two types of gland-cells are also to be found in the ectoderm ; one of 

 these consists of elongated fusiform cells containing many small granules 



Fig. 2. Section through body-wall of Zoanthella, showing char- 

 acter of ectoderm and mesogloea. X333. 



and lying with the outer end near the free border of the epithelium ; the 

 other type consists of rounded cells containing larger granules lying at the 

 base of the epithelium ; these cell granules stain intensely with plasma stains 

 (text-fig. 2). In Zoanthina there are gland-cells which stain intensely with 

 nuclear stains and which constitute a third type (text-fig. 5). 



Other cellular elements of the ectoderm are the Zooxanthellse, which are 

 especially abundant in Zoanthella; these are small round cells with dense 

 nuclei and with yellowish or greenish chromatophores. They are found most 

 abundantly in the outer portion of the ectoderm. 



At the base of the epithelium and adjoining the mesoglcea is a layer of 

 fine fibrils which run in all directions and which consequently appear in sec- 

 tions as fine dots or short fibers. These are probably the fibrillar bases of the 

 epithelial cells. 



The description just given applies to the general ectoderm of the larvae. 

 In the region of the ciliated band this epithelium is remarkably altered. The 

 nematocysts and gland-cells are here lacking, while the ordinary ectoderm 

 cells are replaced by exceedingly slender elongated cells (text-fig. 3). The 

 nuclei of these cells lie in the deeper part of the epithelium, while the cell- 

 bodies consist of slender filaments which are continued from the nuclei to 

 the periphery of the epithelium and then into long flagellje, which constitute 

 the vibratile band. The free border of the epithelium is marked by a faint 

 line. It seems probable that this line marks a plane along which the various 

 cell filaments fuse together. The filaments are apparently as numerous 

 within the epithelium as without it and they are more numerous and more 

 powerfully developed than in any other epithelium I have ever seen. At 

 the base of this ciliated epithelium are a few rounded cells, some of which 



