92 MARGARET 0. COLLCUTT. 



this layer the thickness of the coenosarc varies considerably 

 throughout the colony ; over the tips of the chitinous spiuules 

 and the jagged ridges of the spines this ectodermal layer is 

 often so shallow as to be almost inappreciable. 



The nucleus is large, and situated in the centre, or rather 

 towards one side of the cell ; it is oval in shape, and has a 

 dense, deeply staining reticulum with several nucleoli. 



In some regions of this ectodermal layer the cells are very 

 vacuolated; the nuclei of these cells are easily discernible, 

 and are surrounded by a mass of coarsely granular protoplasm, 

 from which protoplasmic strands extend to the cell borders 

 (PI. 1, fig. 5). 



In other places, however, the cells are closely crowded 

 with deeply staining roundish corpuscles, which are often 

 present in such numbers as to obliterate from view the 

 boundaries between the cells and hide the nuclei. Probably 

 the presence of these corpuscles is accounted for by the fact 

 that the lower ectoderm apparently secretes some of the nema- 

 tocysts which frequently occur in great abundance in these 

 parts of the lower ectoderm (PL 1, fig. 1, c). 



As was mentioned by Strethill- Wright (10), there are two 

 kinds of nematocyst in Hydractinia, small and large j both 

 kinds are found in the lower ectoderm. 



The ectoderm of the gasterozooid tentacles is crowded with 

 the small variety, while large nematocysts occur in the ecto- 

 derm of the blastostyle and dactylozooid tentacles, and in the 

 ectoderm of the tips of the tentacular polyps ; nematocysts 

 are also found in small numbers throughout the ectoderm of 

 the bodies of the gasterozooids, blastostyles, and dactylo- 

 zooids, and in large numbers in the ectoderm of the bodies of 

 the tentacular polyps, but I have never found both the large 

 and small variety occurring in the same individual. 



B. Histology of the Upper Ectoderm. 



1. Ectoderm of the Basal Coenosarc. — This forms a 

 continuous and regular layer of cells, which are approximately 

 equal in size, and appear cubical in section. 



