618 Haddon and Shackleton — A Revision of the British Actinice. 



stead. The infoldings of the endodermal sphincter, especially in its upper portion, 

 are frequently so cut across by the razor in sections as to appear isolated, and thus 

 the muscle might be supposed to be partly mesogloeal in character (PI. LX., fig. 8). 

 It is possible that this may actually occur to a very slight extent. In either case 

 the distinction between Zoantheee with an endodermal and a mesogloeal sphincter 

 is not so fundamental as might appear at first sight. 



All other Zoanthese have a mesogloeal sphincter. In Sphenopus the sphincter 

 is extremely long, as Erdmann has previously remarked ; Zoanthus alone has a 

 double sphincter (PL lxiv., figs. 3 and 5). 



Capitidum.— The capitulum, as all authors have described, is thrown into 

 ridges ; these have a certain amount of specific value, but too much reliance 

 should not be placed upon this character. 



In all species the ectoderm retains its character as a continuous epithelium. 

 In Z. copjjingeri the ridges are crowded with nematocysts, but we do not find this 

 of common occurrence. 



Tentacles. — The ectoderm of the tentacles contains numerous sausage-shaped 

 nematocysts. The deeper layer of the ectoderm usually exhibits a well-marked 

 nervous layer, the nerve-cells of which are shown in PI. lxiv., fig. 2. There is 

 a diffuse ectodermal muscular sheath, the fibres of which have a longitudinal 

 direction. 



The mesogloea is usually thin. The endoderm is relatively thick ; and in Z. 

 copjjingeri, Z. julcesii, andZ. macgillivrayi zooxanthellse are here esjDCcially abundant, 

 but in /. asymmetricus and in Palythoa hoivesii and P. kochii, although they are 

 present in the endoderm of the body-wall, few, if any, are to be found in this region. 

 In Z. coppingeri numerous nematocysts of oval shape, similar to those found in the 

 ectoderm of the body- wall, are present in the endoderm of the tentacles (PI. lxiv., 

 fig. 2). And in some of our specimens of E. couchii similar nematocysts are to 

 be found in the ectoderm of the tentacles. A diffuse endodermal muscular layer 

 consisting of fibres which run in a circular or horizontal direction, and which may 

 be regarded as an extension of the muscular layer of the body- wall, is found in the 

 tentacles of all our species. 



Z>/sc.— The structure of the disc is usually similar to that of the tentacles. As 

 in the latter, there are no incrustations. 



(Esophagus. — The ectoderm of the oesophagus is usually more or less folded ; 

 but as the degree of folding is variable in different individuals of the same species, 

 and probably also in the same individual in different conditions of contraction, 

 this character is of little value for systematic purposes. The same may be said of 

 the nature and extent of the groove. A sulcar groove is always present, but it is 

 scarcely discernible in our specimens of I. asymmetricus^ and in one of the specimens 

 of E. incrustatus (PI. lx., fig. 1). In other specimens of the latter species it is. 



