BIVALVES. 69 
incontestably mechanical as that of a wire-drawer’s 
mill, The Pinna is provided with an extensile 
member, like a finger, and this contains a glue, which 
the animal protrudes at pleasure, through a variety 
of minute perforations in the tip. This glue, or 
gum, as.in the instance of the common spider, or 
the silk-worm, having passed through these apertures, 
becomes threads of almost imperceptible fineness ; 
and these threads, when joined, compose the silk 
which is so much valued by the Sicilians. But the 
animal first attaches the extremity of the thread, by 
means of its adhesive quality, to some crag, or pebble, 
of unusual size; and when this is effected, the Pinna 
receding from that point, draws out the thread 
through the perforation of the extensile member by a 
process, which Paley, in describing the similar opera- 
tions of the terrestrial silk-worm, justly compares to 
the drawing of wire. One difference only subsists. 
The wire is the metal unaltered, except in figure: 
whereas, in the forming of the thread, the nature of 
the substance is somewhat changed, as well as the 
form; for, as it exists within the insect, it is merely 
a soft and clammy glue; the thread acquiring, most 
probably, its firmness and tenacity from the action 
of the air upon its surface at the moment of expo- 
sure. This property is, consequently, a part of the 
contrivance. 
