Haddon and Shackleton — A Revision of the British ActinicB. 617 



are abundant in the Parasoanthus dichroicus, and are also common in P. anguicoma^ 

 and P. doiiglasi. They occur also in Gemmaria macmurrichi, Palythoa howesii, and 

 P. kochii. 



We have no proof that these small and isolated groups of cells, which have 

 been aptly termed "cell-islets" (" Zellinseln ") by Erdmann, are connected in 

 any way with the canals or the lacunae, and, like that investigator, we do not 

 know their origin. We regard these islets as simply groups of ordinary meso- 

 glceal cells. 



Endodermal and ectodermal hays. — We may here refer to the endodermal bays 

 described for Isaurus [_" Mammillifera^'^ tuherculatus by M'Murrich (1889, p. 118); 

 he says: "In some of my sections deep bays can be seen running from the 

 endoderm into the mesogloea, and from their ends and sides numerous canaliculi 

 can be seen branching out. These bays can be found in various states of enclosure 

 by the mesogloea, the cells which they contain being in some cases continuous 

 with the general endoderm, in other cases almost separated from it, and finally 

 quite so. So, too, with the ectoderm." We have found similar deep endodermal 

 bays in Isaurus asymmetricus (PI. lxiv., fig. 9), but in no case were the bays quite 

 separate from the general endoderm. In our species the ectodermal bays (PI. lxii., 

 fig. 4) differ considerably from those of M'^Murrich's species ; the latter we have 

 been able to examine through the courtesy of our friend, and as he has not 

 figured one of these bays we add one for comparison (PL lxiii., fig. 3). 



Endoderm. — The endoderm of the body-wall presents few features worthy of 

 remark. In I. asymmetricus we have found nematocysts smaller than those which 

 occur in the ectoderm. ZooxanthellaB are present in the three species of Zoanthus 

 from Torres Straits, and appear to be characteristic of this genus as well as Isaurus. 

 They are also extremely numerous in G. mutuki^ and are present in Palythoa 

 howesii and P. kochii. In Parasoanthus dixoni the endoderm is thickened into ridges 

 between the mesenteries, but in most cases it is of uniform thickness. 



Diffuse endodermal muscle. — The base of the endoderm forms a feeble but 

 complete muscular sheath ; as the fibres run in a horizontal direction, the muscle 

 is scarcely to be seen in transverse sections ; in vertical sections (PI. lix., figs. 9 

 and 12) they are readily seen. 



Sphincter muscle. — The diffuse endodermal muscle of the general body-wall 

 becomes converted in the capitular region into a sphincter muscle, which in contrac- 

 tion causes the introversion of the corona and capitulum. The genus Parazoanthus is 

 unique amongst the Zoanthese in possessing an endodermal sphincter. This fact 

 was first discovered by Erdmann (1885, p. 468), who made this a primary character 

 in the definition of his interpretation of the genus Palythoa, of which he took 

 P. axinellce as the type. As we shall subsequently explain, Erdmann' s genus 

 Palythoa cannot stand ; so we have erected the new genus of Parazoanthus in its 



TRANS. EOT. DDE. SOC. N.S. VOL. IV., PAET XII. 4 T 



