Stkphknson — Certain Actiniaria collected off Ireland. 129 



mesenteries of all cycles, save the primary, are fertile ; the last (fifth) cycle 

 is not fully formed. There seem to be oral, but no marginal stomata. There 

 is a tendency towards one mesentery of a pair being larger than its partner- 

 in the younger cycles, but it is not well or clearly developed, and in the 

 majority of pairs it would not be possible to say which was the larger, by 

 dissection at any rate. Sections of a sector show that the first three cycles 

 bear weak pennons, the fourth and fifth being reduced to little more than 

 reproductive septa. The mesenteries are very thin, and all have a slight 

 thickening of the mesogloea, and a parietal muscle where they join the body- 

 wall. (PI. XVIII, ng. 10.) 



(a) A tjipieal direr/ in- mrsm/eri/. — The parietal muscle has the processes 

 on the longitudinal-muscle side larger, fewer, and more branched than on 

 the other. Most of the mesentery is very feebly muscular, though there are 

 short, stout processes on the endocoelic side. But as the edge of the mesentery 

 which joins the actinopharynx is reached, a weak diffuse pennon appears, its 

 processes becoming higher and higher till they abruptly end at the juncture. 

 They are very stout and little branched, and crowded, so that they have a 

 somewhat "reversed" appearance (cf. PI. XVIII, fig. 3) (i.e., as if the pro- 

 cesses projected from endoderm to mesogloea, instead of the reverse, which 

 is really the case). 



As to the rest of the mesenteries, the parietal muscle is always better 

 developed on the same face of the mesentery that has the pennon than on 

 the other. The greater part of the mesentery is thin, and feebly muscular, 

 the second and third cycles bearing a feeble pennon at the distal border (at 

 the level sectionized), which occupies only a small proportion of the muscular 

 surface, and tapers at both ends, typically more abruptly distally. The 

 processes are short and stout, and not much branched, giving almost the 

 " reversed " appearance referred to above. The parieto-basilar muscle, at the 

 level sectionized, is weak, and has no free edge, but fringes almost the whole 

 muscular part of the mesentery, often rising into lobe-like processes. 



(ii) Sphincter. — Mesogloeal, rather weak. (PI. XVIII, fig. 6.) It is narrow 

 throughout, widest somewhat below the top, the upper part being mixed up 

 with the tentacle-bases, and so variable in different sections. At its lower 

 end it lies close to, but never in contact with, the endoderm. from which it 

 is separated throughout by a band of mesogloea; above, it approaches fche 

 ectodermal surface of the mesogloea. It ends rather abruptly, botli ahove 

 and below — below the muscle bundles become fewer, smaller, ami more 

 scattered; above (PI. XVIIi, fig. 5) the terminal bundles arc distinct. In 

 structure it is alveolar, the fibres to a large extent being arranged in little 

 rings of variable shape and size as to detail. Throughout the greater pari of 



R.I. A. PKOC, VOL. XXXIV, SECT. IS. [S] 



