THE STRUCTURE OF XENIA HTCKSONT. 267 



Zooxanthellse are exceedingly numerous in the endoderm of 

 the pinnules, so numerous that, in many cases, the lumina of 

 the pinnules are entirely closed. They are also numerous in 

 the endoderm of the tentacles and of the free portion of the 

 polyp, but in the coelentera of the stem there are few, except 

 near the upper eud. There do not appear to be any large gaps 

 between the endoderm cells in the lower parts of the coelentera, 

 as described by Hickson in Alcyonium, but the endoderm 

 cells in this part are more or less spherical in shape, and are 

 only loosely connected together. 



Mesogloea. 



1. Of the Free Portion of the Polyp.— The mesogloea 

 of the body of the polyp varies in thickness from -02 mm. 

 to -06 mm., while that of the tentacles is much thinner, 

 averaging -013 ram. The mesogloea of the pinnules is exceed- 

 ingly thin, especially when they are expanded (cf. figs. 11, 17). 



Cells which connect the ectoderm and endoderm may be 

 seen crossing the mesogloea in all parts of the tentacles and 

 body of the polyps. In the tentacles, however, these cells 

 are few in number, but in the body of the polyp, where the 

 mesogloea is thicker, the cells are more numerous. They are 

 usually elongated or fusiform cells, having their outer 

 ends embedded in or connected with the ectoderm, and their 

 tapering inner ends passing into the endoderm (fig. 17). In 

 most cases the connection between the two cell-layers is estab- 

 lished by means of a single cell, but in some cases two or more 

 cells are placed end to end to form the connecting cord. In 

 most cases the larger portion of the cell and its nucleus are 

 situated in the ectoderm, and this, together with the nature of 

 the cell,which closely resembles an ectoderm cell in appearance, 

 points to the fact that these cells in the mesogloea are derived 

 from the ectoderm. Besides these moderately large cells, the 

 protoplasm of which is finely granular and often contains a 

 few small vacuoles, there are other cells of very much smaller 

 size which bear several or many processes. Some of the cells 



