252 MOLLUSCA. 



but very different manner, being moored like a ship 

 with one or more cables of strong silky threads, 

 called byssus. The common Mussel (Mytilus Edulis) 

 is a familiar example; but, more celebrated for this 

 structure is the genus 



Pinna,* the Wing -shell, 



in shape like a Mussel, but more triangular; 

 though large, being sometimes three feet in length, 

 it is frail, being semi-transparent and very thin. 

 In order to secure it from those bufferings of the 

 storm, which might soon dash its brittle habitation 

 to pieces, the Pinna forms a series of threads of 

 great strength, but of a silky fineness, of which in 

 Italy, valuable stuffs have been woven. These are 

 not spun out to an indefinite length, as in the Silk- 

 worm and Spider, but are formed in a mould. The 

 foot, which is small, has a groove through its length, 

 which by the closing of its edges may be formed 

 into a canal. A glutinous fluid is secreted in the 

 bottom of this groove, which soon hardens into a 

 thread, but before it has become quite hard, and 

 while yet tenacious, the end of the foot is thrust 

 out and applied to the surface of the rock, to which 

 the newly made thread adheres by its extremity ; the 

 foot is then withdrawn, and the canal opened and 

 prepared to make another. Many threads are thus 

 formed in succession, and stretched in different direc- 

 tions, but so slow is the operation, that not more 



* Pinna, a fin. 



