200 J. E. Duerden — Jamaican Actiniaria : 



are rather narrow and deep, but the presence of numerous sponge spicules 

 interferes with a detailed study. As noticed amongst the external characters 

 the polyps are capable of excessive retraction, so much so as to obliterate a 

 great part of the ccelenteric space, and produce a great displacement of the disc 

 and stomodseal walls. 



The ectoderm of the tentacles presents throughout its extent a peripheral 

 layer of small, narrow nematocysts, differing from the oval form in the column- 

 wall. Below this nematocyst layer a nuclear zone is usually separable from 

 the more internal nervous and muscular elements. An ectodermal and an endo- 

 dermal musculature are developed ; the former much the stronger. The layer of 

 nerve fibrils is often distinguishable in connexion with the ectoderm. The 

 mesogloeais very thin, but a little better developed proximally ; it is finely plaited 

 for the support of the ectodermal musculature. 



The endoderm contains zooxanthellse, and very often fine pigment granules ; 

 these latter are also found in the ectoderm. Spicules occur in some abundance 

 in the tentacular tissues, somewhat more numerous in the outer than in the 

 inner cycle. Though such a position for inclusions is exceptional they are 

 met with in all the examples studied, and in such a manner as to leave little 

 doubt that they are not the result of displacement during the preparation of the 

 sections. 



The disc is very thin, and peripherally closely resembles the tentacles in 

 structure ; a few nematocysts occur in the ectoderm, as well as numerous 

 deeply-staining granular gland cells. 



The vertical height of the stomodaeum is remarkably small in contracted 

 specimens (PI. xiv., fig. 4) ; and in a series of transverse sections the sulcar end 

 terminates in advance of the sulcular. The single gonidial groove is clearly 

 indicated. In transverse sections the wall is usually cut through twice as 

 a consequence of the partial reflection of the internal termination of the stomo- 

 dseum. As seen in the figure, the stomodagal wall passes slightly upwards and 

 outwards for a considerable distance. The ectoderm displays the usual histo- 

 logical structure, consisting mainly of ciliated supporting cells, the combined 

 nuclei of which give rise to a very distinct zone ; granular gland cells, which also 

 stain deeply, are abundant, especially in the upper regions, but nematocysts do 

 not appear to be developed. No ectodermal musculature is discernible over any 

 part of the stomodseum. The mesogloea is very thin, and undergoes no appreci- 

 able thickening at the groove. In vertical sections the stoinodasal ectoderm 

 is in continuity with the mesenterial filaments ; but there is no special forma- 

 tion of reflected ectoderm. 



Owing to the extreme retraction and the shortness of the stomodaeuin, some 

 difficulty is experienced in making out the arrangement of the mesenteries ; but 



