Industrial and Manufacturing Uses of Shells. 309 



to be spun from a secretion in the foot. Poli thinks them 

 to be merely prolongations of tendonous fibre. 



The Pinna possesses a machine as incontestibly mecha- 

 nical as a wire-drawer's mill. It is provided with an ex- 

 ternal member like a finger, and this contains a glue, which 

 the animal exudes at pleasure by means of a variety of 

 minute perforations in the lip. This glue or gum, as in 

 the instance of the common spider or the silkworm, having 

 passed through these apertures, becomes threads of almost 

 imperceptible fineness ; and these, when combined, com- 

 pose the marine silk which is so much admired by the 

 Sicilians. 



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 operations of the terrestrial 

 silkworm, justly compares to the drawing of wire. One 

 difference alone exists : 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 water, 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 exposure. 



This byssus forms an important article of commerce 

 among the Sicilians, for which purpose considerable num- 

 bers of Pinna are annually fished up in the Mediterranean 

 from the depth of 20 to 30 feet. An instrument called a 

 " cramp " is used for the purpose. It is a kind of iron fork, 

 with perpendicular prongs eight feet in length, each of them 

 about six inches apart, the length of the handle being in pro- 



