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Part II.—Stichodactylinee and Zoanthee. 165 
The tentacles appear as simple outgrowths of the disc, no special sphincter 
being developed at their origin.. The three layers are about equal in thickness, 
and may be a little folded in preserved material. Nematocysts are only borne 
towards the apex of the tentacles, but there is little or no enlargement distinguishing 
the ecapitulum from the stem. The nematocysts are rather long and very narrow, 
and the spiral thread inside is easily recognized. They are closely packed in a 
peripheral zone; below this is a broad nuclear zone; then a clearly defined thin 
nervous layer ; and, lastly, the longitudinal ectodermal muscle on fine mesogloeal 
plaitings. The endoderm is crowded with zooxanthelle and gland cells; the 
granular contents of the latter are in many cases in the act of being extruded into 
the tentacular cavity. There is only the merest trace of an endodermal muscu- 
lature; endodermal nerve fibrille are distinguishable, but do not unite into a 
distinct layer, as in the ectoderm. 
The ectoderm of the dise is devoid of cnidoysts, but contains numerous 
glandular cells with granular contents. A weak, radial, ectodermal musculature 
occurs, and the circular endodermal muscle is more strongly developed than in 
the tentacles, the mesoglea being deeply plaited; the nerve fibrille are clearly 
seen in places, united into an extremely thin layer some distance from the muscle 
layer. Gland cells are abundant in the endoderm. 
In a dissection of a small specimen, through the middle of the stomodexal 
region, twelve pairs of perfect mesenteries occurred, of which two pairs were 
directives; an alternating cycle of twelve pairs extended about half-way towards 
the stomodzum; and, of the third cycle, made up of twenty-four pairs, some 
extended only just beyond the column-wall, while others were larger. In other 
and larger specimens numerous irregularities were presented, pairs belonging to 
any of the cycles being missing or present in excess. The number of perfect 
mesenteries in these becomes very considerable, appearing as if closely arranged 
all round in alternating perfect and imperfect pairs as described by M*Murrich 
(p. 40). In one example, where the disc was exceptionally transparent, thirty-six 
pairs of mesenteries reaching the stomodzeum could be counted 
The first part of a mesentery is narrow; it then thickens abruptly, the retractor 
muscle extending nearly across the face, again terminating in an abrupt manner 
in the imperfect pairs, but gradually in the perfect. he microscopic appear- 
ance of the retractor muscle is figured by M‘Murrich. All the mesenteries in 
section appear at first undulating on both sides, due to the enlargement of the 
mesoglea, but become straight towards the stomodeum. On the face opposite 
the retractor muscle a thin musculature occurs all along, but the mesoglea is not 
plaited. The parieto-basilar muscle appears developed on this face only. The 
endoderm is loaded with glandular cells, and fine nervous fibrillee occur between 
the musculature and the denser peripheral parts of the endoderm. 
TRANS. ROY, DUB. SOC., N.S. VOL, VII., PART VI. 2B 
