Part II. — Stichodactijlince and Zoanthece. 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 capitulum 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 zooxanthellse 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 endoderrnal muscu- 

 lature ; endoderrnal nerve fibrillar are distinguishable, but do not unite into a 

 distinct layer, as in the ectoderm. 



The ectoderm of the disc is devoid of cnidoysts, but contains numerous 

 glandular cells with granular contents. A weak, radial, ectodermal musculature 

 occurs, and the circular endoderrnal muscle is more strongly developed than in 

 the tentacles, the mesoglcea being deeply plaited ; the nerve fibrillas are clearly 

 seen in places, united into an extremely thin layer some distance from the muscle 

 laver. Gland cells are abundant in the endoderm. 



ml 



In a dissection of a small specimen, through the middle of the stomodseal 

 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 stomodseum ; 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 c Murrich 

 (p. 40). In one example, where the disc was exceptionally transparent, thirty-six 

 pairs of mesenteries reaching the stomodseuro. 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 abiurpt manner 

 in the imperfect pairs, but gradually in the perfect. The microscopic appear- 

 ance of the retractor muscle is figured by M c Murrich. All the mesenteries in 

 section appear at first undulating on both sides, due to the enlargement of the 

 mesoglcea, but become straight towards the stornodseuin. On the face opposite 

 the retractor muscle a thin musculature occurs all along, but the mesoglcea is not 

 plaited. The parieto-basilar muscle appears developed on this face only. The 

 endoderm is loaded with glandular cells, and fine nervous fibrillse occur between 

 the musculature and the denser peripheral parts of the endoderm. 



TRANS. ROY. DUB. SOC, N.S. VOL. VII., PART VI. 2 B 



