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



In Parazoanthus dichroicus cell-islets occur in the mesoglcea of the mesenteries, 

 and also in E. paguripMlus. 



Nematocysts are sometimes present in the canals and sinuses of the mesenteries 

 {e.g. Z. coppingeri (PI. LXii., fig. 1) and Pahjthoa hoioesii)\ but zooxanthellae are 

 I'arely present. We have found them in Palythoa kochii. 



Endoderm. — The endoderm of the proximal portion of the mesenteries usually 

 resembles that of the body-wall, except that it more frequently contains nematocysts, 

 as in Z. coppingeri, Z. macgillivrayi, and /. asyinmetricus. In the latter species they 

 are much smaller than those of the ectoderm of the body-wall. Zooxanthellse 

 occur in the endoderm of the mesenteries in those species in which they are 

 present in the general endoderm. 



M'^Murrich has carefully described the swollen distal portion of the endoderm 

 of the mesenteries of Z. flos-marinus (1889, ]). 115, PI. vii., fig. 4) in that region of 

 the body where the mesenterial filament is simple. We have found a similar 

 enlargement in Z. macgillivrayi (PI. lxiv., fig. 8), in which nematocysts are present. 

 Our specimen differs from his chiefly in the possession of zooxanthellae and 

 nematocysts ; we have not observed in our species that these endodermal cells 

 are "loaded with green granules, closely packed together." 



A similar swelling occurs, to a variable extent, in other species ; for example, 

 in P. axinellce (PI. lx., fig. 7) it is only moderately developed, and we have not 

 observed nematocysts or zooxanthellae ; but these appear to be absent in the 

 general endoderm of this species. Sometimes the swelling is absent, as in E. 

 incrustatus. 



We, too, have found, with M'Murrich, " foreign bodies of organic nature 

 imbedded in the cells, sometimes being surrounded by a number of cells containing 

 no granules, or occasionally imbedded in the mesoglcea" (1889, p. 116). He 

 suggests that these are concerned with digestion (PI. lxiv., fig. 8). 



Muscles. — The muscles of the mesenteries are endodermal and diffuse. As in 

 other Actiniae there are two kinds of muscles, the longitudinal and the parieto- 

 basilar. 



The longitudinal muscle is often very difficult to distinguish in transverse 

 section, being feebly developed, and forming a simple layer of fibres (PI. lxiv., 

 fig. 8). In /. asymmetricus and E. paguriphilus, Parazoanthus dixoni, and others, the 

 muscle is better developed, and is slightly plaited (PI. lx., fig. 9). 



The parieto-basilar muscle is usually relatively broader, and extends higher up 

 the mesenteries than in other Actiniae ; but in Parazoanthus anguicoma and P. dixoni, 

 this muscle is very feebly developed, and only occurs at the insertion of the 

 mesenteries (PI. lx., fig. 9). 



It has been recognized by other investigators that judging by the arrange- 

 ment of the muscles the mesenteries of the Zoantheae are paired, although the 



