370 J. E. Duerden — Jamaican Actiniaria : Parti. — Zoanthece. 



Harbour in water of not more than half a fathom in depth, and only a few yards 

 from the shore. The polyps are very sensitive and active, retracting immediately 

 on being touched. 



Column-wall (PL xx., figs. 1, 2). — The outline of the column-wall, owing to the 

 presence of incrusting material, is very irregular in sections, especially in the 

 lower part ; in the region of the marginal denticulations it is sinuous, and thicker, 

 and the incrusting matter is aggregated opposite the inner circle of tentacles. 

 Where perfect, the ectoderm is continuous ; in most places, it is broken up or 

 absent. It is covered on the outside by a cuticle with an adhering layer of 

 foreign material, mostly diatom frustrules and fine mud. 



The mesoglcea varies in thickness, being much better developed proximally. 

 It contains isolated cells with long processes, cell-islets, and irregular communi- 

 cating canals. The incrustations are sparsely distributed, and are mostly siliceous 

 sand grains and a few sponge spicules. 



The endoderm is very thin and regular, and the transverse muscle is well 

 developed. 



Sphincter muscle (PI. xx., fig. 1). — The single mesogloeal sphincter muscle is 

 small, and formed in a few, irregular, closely set cavities, extending about half 

 way across the mesoglcea, and situated just at the base of the outer row of 

 tentacles. The lining muscle-fibres are weak, and other rounded cells partially 

 fill up the cavities. 



Tentacles (PL xx., figs. 2, 3). — The ectoderm of the tentacles is thick compared 

 with the two other layers, and the ectodermal muscle is well developed on small 

 mesogloeal plaitings ; numerous small oval nematocysts occur, and pigment 

 granules in places. The nervous layer is clearly distinguished between the 

 nucleated zone and the muscle fibres, and sends connecting strands to each. The 

 mesoglcea and endoderm are both very thin. An endodermal muscle layer is 

 present, seen in longitudinal sections. 



Disc (PI. xx., fig. 2). — The structure of the disc is much like that of the ten- 

 tacles, but the ectoderm is not so well developed, and its outer grooved portion is 

 in places loaded with pigment granules. 



(Esophagus. — In extended specimens the oesophagus is almost circular in out- 

 line ; the oesophageal groove is quite shallow. In longitudinal sections the wall is 

 thrown into transverse folds. The ectoderm is a very regular, ciliated layer, with 

 abundant gland-cells and a few small nematocysts; pigment granules occur in 

 the deeper parts ; it is reflected above the lower termination of the oesophagus, 

 and below forms the mesenterial filaments in the usual manner. The mesoglcea 

 and endoderm are very thin, especially the latter. 



Mesenteries (PL xx., fig. 4). — Sixteen pairs of mesenteries, presenting the usual 



