TUBE-MAKERS. 337 



about three inches in length, often attached to old 

 shells, and differs from the other tubicolous annelids 

 enumerated, by the possession of an operculum, with 

 which to close at pleasure the orifice of its tube. This 

 operculum is recognised as a modification of one of the 

 tentacles, the other remaining in its normal condition. 

 The tubes are homogeneous, and of a shell-like 

 substance, sometimes solitary, but often clustered, 

 and intertwined in a contorted manner (hence it has 

 been called Serpula contortuplicatd), wrinkled trans- 

 versely, rounded, with a sharp 



J ffy. 65 



dorsal keel, which is sometimes % \ 



almost or entirely obliterated. 

 The branchial tufts, and also the 

 operculum, are blotched with a 

 beautiful scarlet colour. In the 

 various species there will be 

 found to be considerable varia- 

 tion in the form of the " stopper," 

 or operculum (fig. 65) ; in some 

 species it is discoid, whilst in others it is funnel- 

 shaped, almost like a miniature wine glass, and with 

 the margin either even, or crenate, or lobed, or ele- 

 gantly toothed, whilst in others it is complex, with 

 two or three projecting ridges or galleries superim- 

 posed. The form and character of the operculum has 

 been regarded by some as of primary importance 

 in the classification of genera and species. 



z 



