MODE OF CONSTRUCTING THE TUBE. 291 



surprising elongation of the tube will be discovered ; 

 or perhaps, instead of a simple accession to its walls, 

 the orifice will be surrounded by forking threads of 

 sandy particles agglutinated together. 



The architect has now retired to repose ; but, as 

 evening comes, its activity is renewed, and against 

 sunrise a further prolongation has augmented the 

 extent of its dwelling. 



At first sight the numerous tentacula seem only so 

 many long cylindrical fleshy threads of infinite flexi- 

 bility. On examining them, however, more atten- 

 tively, we see, that 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 different places to hold the particles 

 securely*. 



* 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 

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

 meet, or a semicylindrical 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 



o2 



