THE SHELL-BINDER TEREBELLA. 239 



in exercising their special function, the surface which is applied 

 to the foreign objects becomes flattened into twice or thrice its 

 ordinary diameter ; and while conveying the sandy materials to 

 the tube, these are seized and retained in a deep groove, which 

 almost resembles a slit ; in fact, the tentaculum becomes a flat, 

 narrow riband, folding longitudinally in diflerent places to hold 

 the particles securely. 



' Although these organs, when contracted, are collected into a 

 brush scarcely double the thickness of the animal's body, so 

 enormous is their extensibility, that they can be stretched out to 

 the length of four inches, or half the length of the body, thus 

 sweeping the area of a circle eight inches in diameter. 



' A thin internal coating, resembling silk, lines the whole tube, 

 and at the same time serves as a real cement to unite and 

 strengthen its innumerable parts. This silk-like material is 

 derived from a glutinous slime, which exudes from the surface 

 of the body of the Terebella. 



' Notwithstanding the unrivalled expertness and expedition 

 with which this Annelidan advances its work, it has never been 

 observed to resume possession of its tube when once forsaken* 

 To obtain the shelter of a new dwelling in place of the old, its 

 labours are invariably recommenced from the foundation.' 



' In Terebella nebulosa^ writes Dr. Williams, ' the tentacula 

 consist of hollow, flattened tubular filaments, furnished with 

 strong muscular parietes, each tentacle forming a band which 

 may be rolled longitudinally into a cylindrical form, so as to 

 inclose a hollow, cylindrical space, if the two edges of the band 

 meet, or a semi-cylindrical space, if they imperfectly meet. 

 This inimitable mechanism enables each filament to take up 

 and firmly grasp, at any point of its length, a molecule of sand, 

 or, if placed in a linear series, a row of molecules. But so 

 perfect is the disposition of the muscular fibres at the extreme 

 free end of each filament, that it is gifted with the twofold 

 power of acting on the sucking and the muscular principle. 

 When the tentacle is about to seize an object, the extremity is 

 drawn in, in consequence of the sudden reflux of fluid in its 

 hollow interior; by this movement a cup- shaped cavity is 



