348 J. E. Duerpen—Jamaican Actiniaria: Part I.—Zoanthee. 
to the column. The projections seen on the upper part of the column are shown 
to be due to thickenings of the mesogloea, and contain a cavity lined with cells 
continuous with the endoderm, as is recorded by M*Murrich (p. 192), for the 
Bahaman forms; further, some polyps do not show any of the ectodermal bays 
mentioned by M*Murrich, and by Haddon and Shackleton, while they occur in 
others. 
Sphincter muscle (Pl. xvi. A, fig. 5).—The single mesogleeal 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 
mesoglea. The appearance is figured by M°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 Zoanthidz, 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 mesogloea abundant pigment 
granules occur, along with a few scattered nuclei. The mesogloa 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 mesoglea 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 Zoanthide. It 
contains numerous ordinary zooxanthelle, 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. 
sophagus.—'The cesophagus 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 
cesophageal groove, ‘The ectoderm is ciliated; the small, deeply-staining oval 
nuclei are arranged in a band a little below the surface. Gland cells, and 
