HYDROZOA, SCYPHOZOA, ACTINOZOA, VBRMES WHITELEGGE. 389 



Sphincter Muscle. The single mesoglceal sphincter muscle is 

 well developed (Plate xxvii., tig. 1, m.s.). The muscle cavities are 

 large and arranged in an irregular alternating fashion. 



Tentacles. The ectoderm of the tentacles is crowded with small 

 slightly curved nematocysts (-010 mm. long), among which occur 

 occasional large ones. Zooxanthellae are also very numerous. 

 The longitudinal ectodermal musculature is strongly developed 

 and supported on close-set plaitings of the mesogloea. The meso- 

 gloea is of moderate thickness and contains only small isolated 

 cells. The entoderm is thin. It contains numerous zooxanthellse 

 but no nematocysts. The circular entodermal musculature is 

 moderately strong. 



Disc (Plate xxvi., fig. 2). The disc is traversed by ridges 

 which pass one from the base of each tentacle of the inner and 

 outer rows to the margin of the mouth. In the ridges both 

 ectoderm and mesogloea are somewhat thickened. The ectoderm 

 especially on the ridges contains nematocysts similar to those in 

 the tentacles and also zooxanthellae. In the deeper portion of 

 the ectoderm there occur numbers of small bright refractive 

 (pigment ?) granules. The ectoderm is devoid of incrustations. 



The mesogloea of the disc is thick, and especially noteworthy 

 from the presence in it of numerous large ectodermal muscle 

 cells (fig. 6, ect. m.) These project into the mesogloea so obliquely 

 that in sections they mostly appear as isolated masses which 

 occupy the upper two-thirds of the mesogloea, and extend from the 

 margin of the mouth across the horizontal part of the disc and for 

 a short distance up in its vertical part (fig. 7). 



McMurrich, andHaddon and Shackleton, also describe enclosures 

 in the disc mesoglcea of the species of Gemmaria examined by 

 them. In G. isolata, McMurrich* describes the mesogloea of the 

 disc as being "densely loaded with enclosed cavities containing 

 cells probably ectodermal and muscular," but in his later descrip- 

 tion of G. rusei, D. & M., he says,t "the enclosures in the 

 mesoglcea of the disc which I thought might possibly be muscle 

 cells in isolata, are seen in Rusei to be comparable to the lacunae 

 of the column wall." Again Haddon and Shackleton in their des- 

 cription of G. macmurrichi (page 689), remark that "cell enclosures 

 (similar to those described and figured by McMurrich) are found 

 in the disc of G. macmurrichi," and they also mention the occur- 

 rence of such in G. mutuki. May it not be that in all these cases 

 we have to do as in the species under description with ectodermal 

 muscle cells, and may not the existence of such in the mesoglcea 

 of the'disc be a character diagnostic of the genus? 



* The Actiniaria of the Bahama Islands. Jour, of Morphology, iii. p. 64. 

 t A contribution to the Actinology of the Bermudas. Proc. Acad. Nat. 

 Sci. Phil., 1889, p. 125. 



