Part II. — Stichodactylince and Zoanlhece t 199 



any stain. The matrix is scarcely discernible except at the inner border, where 

 it may sometimes be observed in connexion with a mesentery. Sponge spicules 

 are present in both the ectoderm and rnesoglcea, though not to the same extent 

 as might have been expected from the nature of the commensalism. Lacunae 

 also occur in decalcified specimens, indicating where calcareous sand-grains had 

 been included. The endoderm is a broad layer, crowded with zooxanthellse. 

 No basal musculature has been detected. 



At the boundary of the base and column occurs an expansion of the wall, 

 there being, as noticed amongst the external characters, a slight formation of 

 ccenenchyme (PL xiv., fig. 4). 



The ectoderm of the column-wall is a broad continuous layer, the columnar 

 character of the cells not being clearly indicated in sections. Its internal limits 

 are ill-defined, partly owing to the foreign inclusions tending to break up the 

 layer, and also to the fact that abundant cells pass from it into the mesogloea. A 

 small oval nematocyst, which does not stain, is scattered sparingly and irregularly 

 throughout. 



The mesogloea contains small, rounded or elongated cells with granular proto- 

 plasm, and also cell-islets, not, however, to the same extent as in the mesogloea 

 of the base. Towards its internal border a very irregular, narrow, encircling 

 sinus occurs, and beyond this it is much more homogeneous. Owing to the 

 strongly cellular nature of the outer region of the mesogloea the encircling sinus 

 is not so distinct as in most of the species investigated by Haddon and Shackleton, 

 nor as in P. tunicans, where the mesogloeal matrix is much more uniform. The 

 cells included in the sinus possess very granular protoplasm, and abundant nema- 

 tocysts similar to those in the ectoderm ; these latter are particularly numerous 

 in the distal region of the polyps. 



Numerous cellular connexions can be traced between the irregular internal 

 limits of the ectoderm and those of the encircling sinus. 



The whole mesogloeal layer contains foreign inclusions, more abundant, how- 

 ever, peripherally ; they are mainly calcareous sand-grains which are dissolved 

 out by acids. Silicious sponge spicules are particularly abundant in the upper 

 region of the column, and always remain in microscopic preparations. The 

 sand-grains are more restricted in their distribution to the region of contact of 

 the ectoderm and mesogloea. 



The endoderm of the column-wall resembles that of the base, but above is 

 much thickened between the mesenteries, while it is narrow below. An extremely 

 weak, endodermal musculature extends the whole length of the column. At the 

 capitulum, the mesogloea becomes sinuous in sections, and the muscle fibres are 

 here a little stronger and represent the sphincter muscle (PL xiv., fig. 4). 



The sphincter is of a diffuse endodermal character ; the mesogloeal folds 



2F2 



