34 



by being more or less calcified as well as by having avicularia and ooecia. The 

 possession of an operculum has been rightly regarded as the most important 

 of the characters mentioned here, and it is Iherelbre so much tlie more of interest 

 that ^^'aters' has shown that an operculum is lacking in the nutritive individuals 

 of the genus Buyiila. Nevertheless, that the species of this genus must certainly 

 be regarded as Cheilostomota is evident, not only from the fact that they ])oss('ss 

 the other Cheilostome characters and are very closely related to the Bkelluria and 

 other genera with an operculum, but also from the fact that they all have avicu- 

 laria, ■which always have an operculum. Nitsche^ has pointed out, and other 

 writers confirmed, that the tentacular sheath from the proximal part of the 

 zooecium whence the polypide originates, grows forward as a solid chord, which 

 is fastened to the inner frontal surface of the zooecium in its distal part. Later, 

 this chord obtains an inner cavity, whicli opens outwardly through a slit in the 

 frontal wall of the zooecium, and it is evident from this that the difference be- 

 tween the structure and development of the aperture in the CheilostonuiUt and CJeno- 

 stomata is conditioned by the difi'erent ways in which this chord-shaped formation 

 is fastened to the inner surface of the zooecium. In the Cheilostomata this attach- 

 ment lakes place in a semi-circular line and this results in the formation of a 

 semi-circular opercular valve. As is known'', we can distinguish in the tentacular 

 sheath between two difi'erent regions: the true tentacular sheath, which in the 

 retracted condition of the polypide encloses the tentacles, and a distal region, 

 the vcslihulum, which can be shut off from the true tentacular sheath by a mus- 

 cular segment, the diaphragm, and as we shall see later the vestibulum may even 

 in certain cases have another closing-apparatus j)laced distally to the diaphragm. 

 As I have nowhere found in the literature a satisfactory account of the way in 

 which the vestibulum is connected with the operculum as well as with the aper- 

 ture of the zooecium, I will try to give such here. If we imagine a zooecium with 

 the operculum (|uite open, but with the [lolypide drawn in, we can distinguish in 

 the vcstibuliun between an inner or zocecial, and an outer or opercular portion, 

 of which the lirst is fastened to the edge of the aperture, and the latter to or a 

 little within the edge of the operculum. liesides the two portions mentioned we 

 also have on each side a triangular lateral portion (PI. XIII, fig. 7 a) which con- 

 nects the zocecial and the opercular portions with each other, and which is folded 

 into the vestibulum when the operculum is closed. The comparison between the 

 vestibulum and a valved purse, the two metal guards of which might respectively 

 correspond with the rim of the aperture and the rim of the operculum, which is 



'111, p. 12; ' 8U, p. JJf— >S; ^ 19, p. 272. 



