306 DK. J. E. DUERDEN ON THE 



within the definition of the family Boloceridse. Kwietniewski (1898, p. 394, pi. xxv. 

 fig. 10) has also described and figured the sphincter in Bolocera McMurricU { = Bolo- 

 ceroides McMurriclii (Kwietn.), Carlgren, 1899). 



The occurrence of a perforated tentacnlo-coeloniic septum and sphincter would appear 

 to be somewhat restricted within the Actiniaria. So far, it is known only in the genera 

 Boloceroidcs, Bolocera, and Bunodeopsis. Its presence would seem to be associated 

 with deciduous tentacles, for the phenomenon is characteristic of all the forms above 

 mentioned. 



According to Carlgren (1899), th.e genera Pohjstomidium and Polijopis, described by 

 R. Hertwig in the report on the ' Challenger ' Actiniaria as destitute of tentacles, are 

 really Boloccrce from which these organs have become detached. Haddon (1898) had 

 independently come to the same conclusion. Carlgren suggests that the detachment of 

 the tentacles may be a means of protection to the polyp. 



McMurrich (1893) had already surmised, and later was able to confirm his suspicions 

 by an actual examination of the specimens at the British Museum, that the ' Challenger ' 

 Actinian, lAponema multifommm, is also a Bolocera, from which the tentacles have been 

 detached. 



Disc. 



The oral disc {disc, PL 25. figs. 4 & 6) is thin-walled periphei-ally, and continues tlius 

 until the lips are reached, when it becomes swollen before terminating at the stomodteum 

 {st.). The discal ectoderm contains many clear gland-cells, and the radial muscle- 

 and nerve-layers are comparatively well developed. At the lips the ectodermal cells are 

 much longer, and thin-walled neraatocy.sts occur, similar to those in the tentacular 

 ectoderm, but stinging-cells appear to be absent from the remainder of the disc. 



The discal mesogloea (fig. 6) is very narrow, and may be delicately plaited on both its 

 outer and inner siirfaces. At the lips it becomes much thickened and more strongly plaited, 

 aftording an increased area for the supjiort of the circular endodermal muscle. This 

 latter becomes so strongly developed as to constitute what must be regarded as a special 

 endodermal circumoral sphincter [or. sph., figs. 4 & 6). The action of the sphincter 

 muscle must have become relaxed during the narcotization of the polyps, permitting 

 the radial musculature to act, for in all the preserved specimens the oral aperture is 

 large and rounded. 



A sphincter muscle in this position is very exceptional among the Actinite, but a like 

 concentration of muscle-fibres has been described by the brothers Dixon for B. rerrucosa 

 (1889, p. 322). It may be considered doul)tt'ul as to how far a special muscular develop- 

 ment would be represented when the oral aperture returns to its living, slit-like condition. 



Other than in its concentration of muscle-fibres the discal endoderm presents no special 

 features. Comparatively few Zooxanthellte are present. 



Stomodceum. 

 The stomodaeum is widely open in sections and usually somewhat irregular in outline 

 (figs. 9-15). Its walls are very delicate, measuring only -045 mm. across. It is 



