614 Haddon and Shackleton — A Revision of the British Acti^iice. 



The larval form of Epizoanthus agrees witli Zoanthus, except that the sulco- 

 sulcar lateral mesenteries are perfect. 



In the Zoanthese new mesenteries appear in the sulcar exocoeles in such a way 

 that the mesenteries nearest the sulcus are the youngest, and those furthest from it 

 the eldest. 



In the Sagartidse the new mesenteries appear in pairs in all the exocoeles. 



The mesenterial filaments, the gonads, the mesenteric canals, and the coenen- 

 chyme will be dealt with later on. We will now proceed to describe the structure 

 of the Zoanthese in greater detail in the following order : — 



Body-tvall — ectoderm, incrustations ; mesogloea, cell-enclosures, endodermal 

 bays, ectodermal bays ; endoderm, diffuse endodei'mal muscle, sphincter muscle ; 

 capitulum. 



Tentacles and Disc. 



(Esophagus. 



Mesenteries. — Imperfect mesenteries, perfect mesenteries, reflected ectoderm and 

 mesenterial filaments, mesogloea, canals, endoderm, muscles, gonads. 



Coenenchyme. 



Development. 



Parasites. 



Body-tvall. — Ectoderm. — The ectoderm is very liable to be rubbed off in the 

 incrusted genera ; where present it generally appears as a continuous layer of 

 narrow columnar cells. In the unincrusted genera, in Gemmaria macmurrichi 

 and in Episoanthus payuriphilus, the ectoderm is traversed by strands of 

 mesogloea, which unite to form a layer peripheral to the ectoderm, and which, 

 in some species break up the ectoderm into more or less cubical blocks (PI. lix., 



%-6). 



External to the ectoderm there is always a cuticular layer which may be very 

 thin, and stains of a darker colour (Parasoanthiis dixoni, PL lix., fig. 9), or it may 

 be thick, in which case it rarely stains, and is often impregnated with dirt 

 (^Epizoanthus wrightii, PI. lix., fig. 3). 



As the cuticle is an ectodermal secretion in forms with a continuous ectoderm, 

 and, as the peripheral layer of mesogloea must also be of ectodermal origin, and is, 

 as a matter of fact, often indistinguishable from the cuticle, we do not consider it 

 of any importance to discriminate between them in the forms with discontinuous 

 ectoderm. 



The above-mentioned layer of mesogloea, peripheral to the ectoderm, is that 

 which is called the subcuticle by Andres (1877, p. 222). 



The ectoderm usually contains nematocysts, which M'^Murrich and others have 

 failed to observe. As a rule these do not stain readily. In some species they are 

 clear ; in others — c. g. E. norvegicus (PL lix., fig. 5), where they are, by-the-by, 



