Stephenson — Certain Actiniaria collected off Ireland. 121 



regular mounds, round which the slender branched processes of the pennon 

 are arranged. (PL XVII, fig. 6.) 



The exocoelic surface is fringed by a straight line of transverse fibres, 

 except for a short distance close to the body-wall where there is a parieto- 

 basilar muscle, with short, rather stout processes. It may end as a distinct 

 fold or not. It seems, as it grows, to enclose in the mesogloea of the 

 mesentery little spaces containing remnants of endoderm, and these are 

 fringed with muscle fibres. 



The larger mesenteries differ from these only in size. 



(ii) Sphincter. — Well developed, moderately long and strong. (Fl. XVII, 

 fig. 4.) It lies immediately against the endodermal side of the mesogloea 

 throughout its length, and, except at its widest point, only occupies a small 

 proportion of the thickness of that layer. The widest point is near its upper 

 extremity, and from that it tapers off very slowly and gradually downwards. 

 The lower portion is a clearly defined band, which passes directly into the 

 endodermal circular muscle of the body-wall on its inner side. This part 

 is clearly and beautifully reticular in structure (PI. XVII, fig. 17), except 

 towards its outer edge, where it tends to become looser, and so alveolar. The 

 meshes of the network are fine and very clear. In some sections there is a 

 definitely reticular inner band next to the endoderm throughout the length of 

 the sphincter ; but in all cases either the outer part alone, or else the whole 

 width of the muscle, splits, as it goes upwards, into slightly radiating, sub- 

 parallel vertical layers, separated more or less from each other by clear 

 mesogloea. These layers are alveolar in structure, and not very definitely 

 bounded (PL XVII, fig. 3), but they vary in width and clearness of definition 

 in different sections. 



(iii) Disc and Te?itaeles. — The radial musculature of the disc is well 

 developed, and entirely embedded in the mesogloea. It varies considerably, 

 however, in sections taken from different parts of the disc. One set of 

 sections shows it as an almost continuous band of varying width (PL XVII, 

 fig. 8) near the middle of the mesogloea. The outlines of the band are not 

 very distinct, and sometimes it is broken up into large clumps, sometimes 

 split into two different bands. It is either " alveolar " or " reticulo-alveolar " 

 in structure, the alveoli on the whole being small. The band is slightly 

 interrupted above the insertions of some of the mesenteries, not perceptibly 

 above others. Then in other sections it is much better developed, definitely 

 reticular in structure, with very large meshes (see I'l. XYII. tig. 7, and note 

 that it is on approximately the same scale as fig. 8), and is sharply inter- 

 rupted over the insertions of some of the mesenteries. In still other sections 

 the meshes are intermediate in size Unfortunately these three sets of 



R.I.A. PROC, VOL. XXXIV, SECT. B. [H] 



