22 SIDERASTREA RADIANS. 



feebly developed. In the column and disc it appears only as a thin dividing 

 lamella between the ectoderm and eudoderm, but in the mesenteries it 

 becomes a little broader. It is homogeneous in structure, except for the 

 presence of minute connective-tissue cells. The mesoglcea of corals gener- 

 ally appears perfectly homogeneous, but in several West Indian species the 

 layer is found to contain migrant connective-tissue cells, such as occur in 

 the larger actinians. 



The endoderm of the column and disc is a little broader than the ectoderm, 

 and its cells contain numerous Zooxanthellae, which are altogether absent 

 from the latter. The endodermal cells are much vacuolated, and distinct 

 cell outlines can be made out. The commensal algae are more numerous in 

 some regions of the endoderm than in others, though there seems to be no 

 regularity in their distribution. Where they are absent, or nearly so, the 

 endoderm is somewhat thinner, its cells are less vacuolated, and the layer as 

 a whole stains more intensely, the nuclei forming a more regular band. 



A delicate circular muscle layer can be discerned over the inner face of 

 the mesoglcea of the column and disc, but no concentration, such as can be 

 regarded as constituting a sphincter muscle, occurs at any region of the 

 column. When studying the living characters it was found that the polyps 

 are unable to fold the column over the disc, hence the presence of a special 

 circular or sphincter muscle, as met with in most anemones and some few 

 corals, would be scarcely expected. 



In sections which have been passed through Delafield's hsematoxylin 

 the deeper part of the endodermal layer throughout the polyp, but particu- 

 larly the epithelial lining of the mesenteries, contains some substance which 

 stains intensely, and has a very irregular distribution. The appearance, as 

 seen in sections of mesenteries, is represented on plate 8, fig. 50, and in the 

 lining of an interseptal loculus on plate 8, fig. 47. Small, irregular, deeply 

 staining patches lie next the mesoglcea on both sides, and prolongations 

 extend for varying distances among the endodermal cells, occasionally 

 reaching almost to the surface. In tangential or oblique sections through 

 the endoderm an appearance of somewhat irregular longitudinal canals is 

 presented. The substance stains and behaves altogether in the same manner 

 as the contents of the mucous cells. No evidence of structure is presented, 

 nor is there any cellular character suggested. I conceive that the appearance 

 is due to intercellular spaces filled in the living condition with mucus or 

 some similar substance, capable of staining strongly in haematoxylin ; per- 

 haps a hint of the lymphatic spaces of the higher animals. 



This is the first time that such a system of apparently intercellular 



