348 J. E. Dueeden — Jamaican Actiniaria: Part I. — Zoanthece. 



to the column. The projections seen on the upper part of the column are shown 

 to be clue to thickenings of the mesoglcea, and contain a cavity lined with cells 

 continuous with the endoderm, as is recorded by M c Murrich (p. 192), for the 

 Bahaman forms ; further, some polyps do not show any of the ectodermal bays 

 mentioned by M c Murrich, and by Haddon and Shackleton, while they occur in 

 others. 



Sphincter muscle (PI. xvm. A, fig. 5). — The single mesoglceal sphincter muscle is 

 strong, elongated, and in longitudinal sections extends nearly across the mesoglcea. 

 Proximally, the mesoglceal cavities are small and circular ; distally, they are oval 

 and more elongated ; the muscle cells are arranged in different directions, and 

 constitute a very thin layer, the remainder of the cavity being partially occupied 

 with loose rounded cells, or more usually appears as an empty space. For the 

 greater part of its length, the muscle cavities give a vesicular character to the 

 mesoglcea. The appearance is figured by M c Murrich, but the cavities appear 

 more numerous and not so elongated in the Jamaican form. Some of my 

 preparations show the constriction he refers to, but I have not obtained the 

 long, branching, terminal cavities. 



Tentacles. — The tentacles, seen in longitudinal sections, are as long as usual 

 in the Zoanthidse, and are acuminate in form. The ectoderm is very thick and 

 shows a peripheral zone of colourless, narrow nematocysts and gland cells ; below 

 this a zone of deeply staining small nuclei ; nearer the mesoglcea abundant pigment 

 granules occur, along with a few scattered nuclei. The mesoglcea is thin, but 

 thrown into fine, long, branching plaits on the ectodermal side for the support 

 of the muscle, to such an extent that, in some sections, the mesoglcea appears as 

 if it had enclosed parts of the ectoderm. This condition is also described for 

 I. asymmetricus (1891, p. 685). The endoderm, even in the state of retraction, is 

 much thinner than the ectoderm, an unusual condition in the Zoanthidse. It 

 contains numerous ordinary zooxanthellas, and occasionally others with thick, 

 highly refractive walls, such as are found in the ectoderm of the body-wall, and 

 are there also associated with the thin-walled form. 



Disk. — The disk much resembles the tentacles, but the ectoderm is not so 

 thick, nor nematocysts so abundant. The nuclei are more uniformly distributed ; 

 but the peculiar ectodermal musculature is similar in places. An endodermal 

 muscle also occurs. 



(Esophagus. — The oesophagus is considerably folded ; the three layers maintain 

 a somewhat uniform thickness, but the mesoglcea follows in places the more 

 deeply folding ectoderm. It is slightly truncated opposite the sulcar directives, 

 the two mesenteries extending from each corner being the only indication of an 

 oesophageal groove. The ectoderm is ciliated ; the small, deeply-staining oval 

 nuclei are arranged in a band a little below the surface. Gland cells, and 



