PLTJSTRELLA. 505 



kind of external operculum, wliicb, though not so highly 

 organized as that of the Cheilostomata, bears some analogy 

 to it, and is very different from the ordinary Ctenostoma- 

 tous structure. The orifice in Fhistrella, which is placed 

 at the very top of the cell, resembles exactly the opening 

 of a common clasp-purse. It is bounded above and below 

 by narrow horny ribs, which correspond with the metal 

 clasps of the purse, and which are connected at the sides 

 much in the same way as the latter, and so as to allow of 

 their opening and closing. When the polypide is about 

 to issue from its cell the mouth opens, just as the purse 

 does, by the separation of the two ribs (or lips), which 

 come together again on its retreat. The upper lip seems 

 to be fixed, and is very slightly arched ; the movable por- 

 tion appears to be the lower lip, which, though not fur- 

 nished with a hinge, like the operculum of the Cheilosto- 

 mata, bends outwards to allow of the passage of the 

 polypide. In this respect, of course, the analogy with the 

 purse fails. When the orifice is fully open, it is some- 

 what quadrangular in form. This arrangement is replaced 

 in Alcyonidium by the simple inversion of the oral appa- 

 ratus, giving rise, during retraction, to the formation of a 

 papillary eminence, puckered in the centre, on the surface 

 of the zoarium. As the polypide of Flustrella issues, a 

 stout cylinder is first protruded, which often remains ex- 

 serted in spirit specimens. 



In another point the genus exhibits some divergence 

 from the rest of the present suborder. The setose operculum 

 is much less conspicuous than it usually is, and, indeed, is 

 often difficult to detect ; but it exists, and is composed of 

 numerous small and slender bristles. The embryology of 

 the genus presents some interesting points. It has been 

 recently investigated by Barrels and Joliet, who have 

 demonstrated the somewhat complex organization of the 



