Part II. — Stichodactylince and Zoanthew. 189 



supporting it become deeper and deeper to give increased area for its support, 

 until they may ultimately unite at their free edges, the muscle thus passing from 

 the endodermal to the mesoglceal stage. 



In P. Siviftii, where the general endodermal musculature and the sphincter 

 are comparatively well developed, small portions of the latter do actually appear 

 to become cut off from the endoderm and become wholly included within the 

 mesoglcea (1898, pi. xx.,fig. 5). The mesoglceal bays are, however, so deep as to 

 suggest the possibility that the appearance of included muscle fibres may be 

 merely a result of the direction in which the section is taken. Such would be the 

 case if the depressions were deep and oblique to the plane of the section. The 

 endodermal muscle is, of course, mesoglceal at the origin of a mesentery in the 

 column-wall. 



Concerning the sphincter of P. dichroicus, Haddon and Shackleton (1891 a, 

 p. 699) remark: — "Near the upper extremity (in contracted specimens), it 

 appears to become embedded in the mesoglcea, a few simple cavities being visible 

 in our sections." We thus possess in the genus, indications, at any rate, of how 

 the actual transference from an endodermal to a mesoglceal muscle is effected 

 (cf. Haddon, 1898, p. 432). 



The musculature is everywhere very feeble in P. separatus, and the sphincter 

 certainly remains entirely endodermal. The polyps of P. monostichns are only 

 about half the size of the former ; and such a muscular weakness is indicated in 

 all the organs that, independently of the interference of the incrustations, the 

 sphincter would probably be difficult of recognition. 



Two of the four Antillean species of Parazoanthus which I have examined, 

 exhibit in their mesenterial filaments a simpler condition than that characteristic 

 of other Zoanthids. 



The structure of the upper part of the Zoanthean filament is well known. It 

 is trifid or V-shaped in transverse section. A middle apical portion of ciliated 

 supporting cells, granular gland cells, and nematocysts, constitutes the glandular 

 streak, Drtisenstreif , or Nesseldriisenstreif ; the outer layer of the two lateral com- 

 ponents consists entirely of narrow, ciliated, supporting cells, and forms the ciliated 

 streak or Flimmerstreif. Coming between the Flimmerstreif and Driisenstrief 

 on each side is a tissue more nearly resembling the ordinary endoderm, and 

 described by Professor von Heider (1895, p. 127 and fig. 16), as the " Entoderm- 

 wucherung." I do not, however, regard it as homologous with the thickening of 

 the mesenterial endoderm immediately behind the filament in its simple form as 

 von Heider appears to (cf. his fig. 28). 



Though differing in form the Zoanthean filament accords in histological detail 

 with that of most other Actiniaria ; the " Entodermwucherung," corresponding 

 with what I have termed the "intermediate streak." 



TRAMS. RCTC. DUB. SOC, N.S. VOL. VII., PART VI. 2 E 



