Hapvpon—The Actiniaria of Torres Straits. 427 
Body-wall.—The ectoderm of the body wall usually has a scalloped appearance 
due to contraction; clear, somewhat pear-shaped cells are very numerous. ‘The 
mesogloea is not as thick as the ectoderm, and is slightly implicated in the ecto- 
dermal scallops; it stains deeply. The endoderm is as thick as the ectoderm 
and mesogloea together, and is crowded with zooxanthelle. 
Sphincter.—The diffuse endodermal sphincter is very feebly developed (PI. 
Xxvil., fig. 3). It consists of about a dozen plaits, which are mostly quite simple. 
Acrorhagi.—The marginal spherules are simply vesicular prolongations of the 
capitulum, and possess no distinctive features; no traces of pigmentation occur in 
the ectoderm in the sections. 
Tentacles.—The ectoderm and endoderm are of about equal thickness; the 
mesoglea is slightly plaited on its ectodermal surface. The ectoderm stains 
deeply, but the nervous basal layer is clear; the nematocysts are small. The 
endoderm contains numerous zooxanthelle. 
The ectoderm of the cesophagus is thrown into numerous folds; the cells are 
granular and ciliated, especially in the grooves, which are variable in number. 
Mesenteries—The mesogloa of the mesenteries is relatively thick; a strong 
prominent retractor muscle is present (Pl. xxvii, fig. 4); the plaitings are long 
and usually slightly branched: the basal muscle fibres on the same side are but 
slightly developed. On the opposite side of the mesentery the strong parieto- 
basilar muscle is produced into a well-developed pennon. 
The filaments which are only borne by the perfect mesenteries are continuous 
with the ectoderm of the cesophagus. The cesophageal grooves are continued a 
long way down towards the base of the polyp. 
The number and arrangement of the mesenteries is very varied, as is shown in 
the following table :-— 
2 grooves, 2 pairs of directives. 14 pairs of non-directives, = 16 
3 ” 3 ” ” 10 ) ” =18 
3 ” 4 ” ” 14 ” ” = 18 
or to write it in another way, 
D+6+d+24+d+6=16* 
D+44D4+24+D+4+4=138 
D+24+D+24+D4+84+D4+7=18 
The character and size of the several mesenteries are indicated in the figures 
(Pl. xxvu., figs. 5, 6, 7). The directives D' in fig. 5 enclose two other pairs 
* In this formula d stands for one mesentery of a pair of directives. 
3R2 
