239 
THE SHELL-BINDER TEREBELLA. 
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 tentacuium becomes a flat, 
narrow riband, folding longitudinally in different places to hold 
the particles securely. 
4 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. 
4 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. 
4 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.’ 
4 In Terebella nebulosa ,’ writes Dr. Williams, 4 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 ydth 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 
