Part II — Stichodactylince and Zoanthece. 171 



the disc, gland cells occur, and the endodermal muscle is much stronger than in 

 the tentacles. 



The stomodseum is greatly folded in all the sections, the ectoderm being 

 followed by long, narrow or broad processes of mesoglcea. The first layer is 

 densely crowded with large, granular gland cells extending completely across, 

 but stinging cells are rare. A very delicate nerve layer can be discerned, and 

 the merest trace of ectodermal and endodermal musculatures. Zooxanthellse are 

 scarce in the endoderm, but gland cells are numerous. 



The gonidial grooves are interesting in the amount of histological detail 

 indicated, and remarkable for the enormously exaggerated endoderm (PI. xiv., fig. 2). 

 The ordinary stomodseal ectoderm and mesoglcea narrow just before reaching the 

 groove, and then all the three layers become much thickened, the endoderm most 

 so. In the ectoderm, the nuclei are nearly all restricted to a narrow, extremely 

 well-defined zone, a little below the ciliated margin ; for a short distance within 

 this zone the layer is almost clear, and then another nucleated zone is apparent, 

 but in this case the nuclei are much fewer and do not stain so deeply. Then 

 comes another clear zone, and afterwards a nervous layer from which fibrillar 

 extend to a very feeble muscle layer, apposed to the inner face of the mesoglcea. 

 Ganglionic cells are scattered here and there among the fibrillas. The whole 

 succession of details can be easily traced all round the gonidial ectoderm. The 

 mesoglcea is smooth on its ectodermal aspect, but the endodermal aspect is 

 irregular ; it is finely fibrous in structure, and many isolated cells are included. 



The endoderm of the groove is enormously swollen, and of peculiar structure. 

 Nearly all the nuclei and protoplasmic contents are aggregated towards its 

 periphery, the greater portion of the layer appearing highly reticular in section ; 

 granular gland cells are scattered about, more numerous towards both its internal 

 and external limitations. The mesoglcea of the directive mesenteries as it passes 

 through the endoderm is extremely narrow. 



Several specimens dissected transversely exhibit numerous pairs of mesenteries, 

 arranged in four orders. The number is very variable, no two of the examples 

 being alike. Two gonidial grooves and two pairs of directives were, however, 

 present in each case. To the naked eye both sides of the groove are smooth, 

 and readily distinguished from the rest of the stomodeeum by being unfolded ; 

 the mesoglcea and endoderm are also much thickened. Twelve pairs of perfect 

 mesenteries were present in a transverse dissection through the middle stomodseal 

 region of a rather small polyp, and also second and third imperfect cycles. In 

 places these exhibited the normal regularity, but in some of the exocceles additional 

 imperfect pairs belonging to lower cycles occurred, and all stages in the develop- 

 ment of new pairs could be traced. In another polyp between thirty and forty pairs 

 of perfect mesenteries were counted in sections through the middle stornodseal region, 



