184 Papers from the Marine Biological Laboratory at Tortugas. 



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

 of these parallel flagellje, 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 la^'er may represent processes from one or 

 both of these kinds of cells. 



The epithelium of the pharyn.x is more compact than that of the general 

 ectoderm and it contains a larger number of gland-cells; consequently 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 layer in which 



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

 ectoderm, mesoglcea, 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 Zoanthella, and along the free border they form a 

 definite epithelium, but in the deeper portions of the la^er there are few 

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

 wandering or amaboid 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. 



