I 4 Papers from the Marine Biological Laboratory at Torttigas. 



flagellum. The iridescence of the ciHated band is due to the great number 

 uf these parallel flagellse, which diffract light like the lines of a fine grating. 



I have not been able to distinguish muscle or nerve cells in the ecto- 

 derm, though the basal fibrillar layer may represent processes from one or 

 both of these kinds of cells. 



The epithelium of the pharynx is more compact than that of the general 

 ectoderm and it contains a larger number of gland-cells; consequenth- it 

 stains more deeply than does the general ectoderm. 



Endoderm. With the exception of the mesenterial filaments the endo- 

 derm stains less deeply than does the ectoderm and the cell outlines are less 

 distinct. Indeed, the general endoderm consists of a spongy layr in which 



Fig. 4. Cross-section through body-wall of Zoanihella, showing 

 ectoderm, mesogloea, endoderm, and two septa with mesen- 

 terial filaments; the gland-cells of the latter are especially 

 evident. X 333- 



are numerous nuclei near the free border, and zooxanthelhe in the 'deeper 

 portion of the layer, but in which cell boundaries are not distinct, except 

 close to the free border. In Zoanthina the cell boundaries and nuclei are 

 more distinct than in ZoanthcUa, and along the free border they form a 

 definite epithelium, but in the deeper portions of the layer there are few 

 nuclei and no cell outlines save those of the symbiotic algae, and of a few 

 wandering or amceboid cells (text-figs. 4 and 5). 



The mesenterial filaments are rich in gland-cells filled with a granular 

 secretion, while adjoining epithelial cells contain no granules, text-fig. 4. 



