302 INTRODUCTION TO CONCHOLOGY. 



changed, as well as the form ; for, as it exists within the 

 worker, 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 property is, consequently, a part of the 

 contrivance. 



The mechanism itself consists of the extensile member 

 which the animal propels at pleasure, of the reservoir in 

 which the glue is collected, and of the external holes com- 

 municating with it ; while the action of the machine is 

 seen in forming the thread, analogous to that of making 

 wire, by forcing the prepared material through holes of 

 proper dimensions. The secretion is an act too subtile for 

 our discernment. But one thing answers to another — the 

 secretory glands to the quantity and consistence required 

 in the secreted substance, and the reservoir to its reception ; 

 while the outlets and orifices are constructed, not merely 

 for relieving the reservoir, but for manufacturing its con- 

 tents into a form and texture of great external use to the 

 life and functions of the Pinna. Moreover the texture is not 

 only essential to the welfare of the inhabiting mollusk, it also 

 constitutes an important article of commerce among the 

 SiciHans ; for which purpose considerable numbers of 'Pinnm 



