Part II. — Stichodactylince and ZoanthecB. 195 



that the calcareous incrustations are limited towards the periphery of the wall, 

 while the siliceous sponge spicules and a few Radiolarian tests are more internal 

 and more distal. 



The endoderm cells are loaded with zoosanthellse. Proximally there is only 

 a faint indication of an endodermal circular musculature, but towards the apical 

 region the fibres become stronger and more concentrated, and constitute a weak, 

 diffuse, endodermal sphincter muscle, the mesogloea forming deep, closely-arrano-ed 

 bays for its reception. No part of the muscle, however, becomes actually enclosed 

 in mesogloea, except at the places where the mesogloea of the mesenteries is united 

 with that of the column-wall (PI. xiii., fig. 7). 



The ccenosarc surrounding the Hydroid stem, and connecting one polyp with 

 another, contains inclusions similar to those of the column-wall. Irregular channels 

 with a thick lining of endoderm serve as a means of communication between the 

 coelenteron of one polyp and another. In sections the Hydroid stem is completely 

 embedded in mesogloea ; this latter also contains abundant cells and cell-islets. 



The ectoderm of the tentacles discloses a peripheral zone of small, narrow 

 nematocysts throughout its length. The mesogloea is thin and very slightly 

 plaited for the sujiport of a weak ectodermal and endodermal musculature, and a 

 nervous layer connected with the former is distinguishable. The endoderm 

 is loaded with zooxanthella?, and completely fills the lumen in contracted 

 tentacles. 



The disc is extremely thin-walled, but becomes a little thicker near the 

 tentacular region, where nematocysts and gland cells occur. 



The stomodaium is of small vertical extent. A single gonidial groove is 

 indicated, very shallow in some examples and deeper in others, while the 

 walls rarely display any vertical folding. The ectoderm is constituted of the 

 usual ciliated supporting cells, granular gland cells, and but few nematocysts ; in 

 the region of the groove, glandular cells are very scarce. An ectodermal and 

 endodermal musculature can be made out, though but feebly developed ; an ecto- 

 dermal nervous layer is also displayed. The mesogloea is much thinner than 

 the ectoderm, and undergoes no additional thickening at the groove. The 

 endoderm is broad, and its cells contain many zooxanthellse. 



At its lower termination the wall of the stomodseum is backwardly and out- 

 wardly directed for a short distance; and the ectoderm is in continuity with the 

 tissue of almost exactly similar nature which runs radially along the edge of the 

 perfect mesenteries, and, as the "Reflected ectoderm," passes for a very short 

 distance up each face of the perfect mesenteries. 



The reflected ectoderm is not developed to the same extent ozi all the 

 mesentei'ies, and very rarely presents a similar appearance on the two faces of the 

 same mesentery. It is constituted of extremely narrow ciliated supporting cells, 



