THE SEETULARIID^E. 



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the stems and roots, but have also a hard, transparent, horny-looking structure which environs the 

 top of the polype and protects the tentacles, are the Campanularia. The hydranths thus furnished 

 can retract almost completely in this calycle, and hence they are called Calyptoblastea. The gonophores 

 arise regularly from the gono- 

 somes, which have neither ten- 

 tacles nor mouth, and some are 

 sessile, and others become free 

 medusae. Most of these medusae 

 have marginal vesicles, and 

 produce the sexual elements 

 in the radial canals. The caly- 

 cles take on most graceful 

 forms, resembling little vases, 

 and often have ornamented 

 borders. The horny cup of the 

 hydranth or nutritive individual 

 or part is called the hydrotheca, 

 and that enclosing the genera- 

 tive buds is called the gonan- 

 gium. 



In many species the aper- 

 ture is furnished with an 

 operculum, which opens to 

 allow of the passage of the 

 polypite, and closes on its 

 retreat. It is a very effective 

 contrivance, and exhibits two 

 or three principal modifications. 

 In some instances the margin is 

 cleft into a number of pieces, 

 which converge and meet in a 

 point, and form a more or 

 less conical lid. In others the 

 cover is amembranous extension 

 of the walls of the calycle, 

 which falls into plaits or folds 

 when the polypite withdraws, 

 and so roofs over the opening. 



STRAW TUBULARIA (Tubularta indivisa). 



FAMILY SERTULARIID^E. 



Amongst the Sertulariidse the calycle has a lid or operculum within it, a little below the orifice. 

 It is attached to the interior surface, on one side, and seems to be a continuation of the inner layer. 

 It shuts down over the polypite when it withdraws itself. When the polypite emerges, it slowly 

 pushes the valve back, and keeps it erect so long as it is exserted ; on its retreat, which is as quick as 

 light, the lid flies back to its place. (Hincks.) This family has the hydrothecae sessile, and more or 

 less inserted in the stem and branches. The polypites are completely retractile, and have a single 

 wreath of filiform tentacles round a conical proboscis, and the gonozooids are always fixed. 



The great Tooth Coralline is one of these, and belongs to the genus Sertularella. The plant- 

 like, branching, jointed stem is rooted by a creeping stolon, and the calycles are decidedly alternate, and 

 have a toothed orifice and a convergent operculum. The reproductive calycles (gonangia) are always 

 more or less ringed transversely. It is of a bright straw colour when living, and is a common shore and 

 deep-water species.* The Tricuspid Sertularella is of a delicate habit, light brown colour, and grows 



275 



Sertularella polyzonias. 



