i44 J. E. Duerden — Jamaican Actiniaria : 



still remains attached to the stomodseum and hangs freely from it, while the 

 longer portion is connected with the column-wall and appears as a free mesentery. 

 Histologically, the stomodseal ectoderm consists mainly of ciliated supporting cells 

 and long, narrow gland cells with finely granular contents ; the nuclei of the 

 former give rise to a brightly-staining middle zone. A weak longitudinal 

 ectodermal musculature and a nervous layer are distinctly recognizable. Nemato- 

 cysts cannot be detected in sections. The mesoglcea is folded on its endodermal 

 border for the support of the weak endodermal muscle. The endoderm itself is 

 an extremely narrow layer, displaying nerve fibrillse. 



In polyps small enough to be sectionized as a whole, twelve pairs of perfect 

 mesenteries are present, of which two pairs are directives. These constitute the 

 first and second cycles ; and it is usually found that the members of the second 

 cycle become free in advance of the others ; the directives, as already stated, 

 extend much below the others, retaining their attachment to the prolonged 

 gonidial grooves. A third cycle of twelve mesenteries alternates with the com- 

 plete members, and extends some distance within the coelenteron, while a fourth 

 cycle of twenty-four mesenteries extends but a little beyond the column- wall. In 

 dissections of large polyps, a fifth cycle of forty-eight pairs of mesenteries occurred, 

 the fourth cycle now stretching well within the coelenteron, and the third cycle 

 becomes connected with the stomodseum for some distance. Trilobed mesenterial 

 filaments are borne by the members of the first three cycles. The mesenteries 

 are exceptionally narrow, and, as a whole, occupy but a small proportion of the 

 coelenteron. The perioral stomata are very pronounced, but the marginal stomata 

 are small and not readily distinguished. 



The retractor muscle is arranged on very narrow, bifurcating, mesogloeal pro- 

 cesses and gives rise to a thickened band, distally situated at a considerable 

 distance from the column-wall (PL xi., fig. 1), but nearer proximally. In sections 

 it commences as a rounded or acute enlargement, and centrally diminishes gra- 

 dually. The musculature beyond the region of the retractor is very feeble, 

 except towards the origin of the mesentery in the column-wall, where, in 

 the region below the stoinodseuni, a parieto-basilar muscle is well developed, and a 

 strongly plaited pennon is present. The endodermal muscle is continued between 

 the mesoglcea at the base of the mesentery, and that of the column-wall. The 

 retractor is developed on all the cycles of mesenteries except the lowest ; while in 

 the proximal region a muscular layer is continued all round the smallest mesen- 

 teries, the mesoglcea being folded. The oblique musculature is very feeble. 



The mesenterial epithelium is narrow, except towards the origin of the mesen- 

 teries, where it broadens, and what appears to be nerve fibrillee become very 

 obvious; the cell contents are here aggregated mostly peripherally (PL xi., fig 1). 



For a short distance both above and below the inner termination of the 



