160 J. E. Duerden — Jamaican Actiniaria : 



The stomodseal ectoderm is deeply folded, a salient feature being the com- 

 paratively large mesogloeal processes, resembling those of the column-wall, which 

 support the folds. As shown in the section figured (PI. xi., fig. 6), these bear no 

 relation to the attachment of the mesenteries on the inner side. There is no 

 modification in structure indicating gonidial grooves. Large nematocysts, such 

 as are met with in the mesenterial filaments, occur sparingly in the ectoderm, and 

 a feeble ectodermal musculature can be discerned. 



The mesenteries are very irregular in their development, but the hexameral 

 condition is evidently the normal ; one or more incomplete pairs may occur in the 

 exocceles between the perfect pairs, or a pair may consist of one perfect and one 

 imperfect mesentery. This irregular arrangement is consonant with what has 

 been already noted for the tentacles, and is likewise probably connected with the 

 usual method of reproduction by fission. 



At least three distinct orders are indicated in most polyps, though many of 

 the members of the third order may be wanting. The section (PI. xi., fig. 6), 

 of a young specimen shows thirty-six mesenteries of varying degrees of 

 development ; of these only thirteen reach the stomodseum. Another presented 

 twenty-four pairs, of which eight pairs were perfect, though in two pairs one 

 mesentery in each fell short of the stomodseum. A very small example sectionized 

 exhibited only seven complete mesenteries, all arising from a region embracing little 

 more than one-half of the circumference of the column- wall, while those arising 

 from the remainder were all incomplete. The mesenteries become very numerous 

 and closely arranged in large specimens. 



Another polyp, sectionized later, is diagrammatically represented in transverse 

 section in fig. 1, PL xn. ; two pairs of directives occur, with three pairs of com- 

 plete mesenteries on one side and four on the other. The members of the third 

 order are present in some of the exocceles, but never in two pairs, one on each 

 side of the pairs of the second order, as in the case where the cycles are deve- 

 loped regularly. 



The retractor muscles, though feeble, are sufficiently well developed to allow 

 of the arrangement in pairs being easily followed. No directives were distin- 

 guishable in two young specimens, but in another two parrs occurred. The parieto- 

 basilar muscles are very distinct on each face, but the mesoglcea is smooth, and 

 affords no indication of any basal pennon. The retractor muscles are supported, 

 in places, on rather considerable plaitings of the mesoglcea, but the distribution of 

 the plaitings is very irregular, and rarely presents the same appearance on any 

 two mesenteries. In some cases they may form two or three thickened vertical 

 bands. In the mesentery represented in fig. 2, PI. xn., only one of these was present. 

 Beyond the retractor region, the mesoglcea becomes extremely thin, sometimes 

 appearing to originate from the side of the thicker part, instead of being a con- 



