CONCHIFERA. 883 



scarcely expect animals so unwieldy to be capable of executing. 

 For this purpose the end of the foot is bent, and placed firmly 

 against the plane of support in the position represented in 

 Jig. 181 ; when thus fixed, a sudden spring-like action of the 

 muscles of the foot throws the cockle into the air, and, by a repe- 

 tition of these exertions, the creature can skip about with surpris- 

 ing agility. 



(417.) But the most extraordinary office assigned to the foot in 

 the class under consideration, is the manufacture of horny threads, 

 whereby, as by so many anchors, the Mollusca thus provided 

 fix themselves securely to foreign bodies, and that so firmly, that 

 extraordinary violence is requisite to wrench such animals from the 

 place where they have fixed their cables. The marine Mussel is a 

 well-known example of a byssiferous Mollusk, and from this species, 

 therefore, we shall draw our description of the organs by which the 

 tough filaments referred to are secreted. 



The foot in the Mussel is of small dimensions, being useless 

 as an instrument of progression. By its inferior aspect it gives 

 attachment to the horny threads of the byssus, which are individu- 

 ally about half an inch in length, or as long as the foot itself, 

 by which, in fact, they are formed, in a manner quite peculiar to 

 certain families of Conchifera ; no other animals presenting a se- 

 creting apparatus at all analogous, either in structure or office, to 

 that with which these creatures are provided. The manner in 

 which the manufacture of the byssus is accomplished is as fol- 

 lows : A deep groove runs along the under surface of the foot, 

 at the bottom of which thin horny filaments are formed by an 

 exudation of a peculiar substance, that soon hardens and assumes 

 the requisite tenacity and firmness. While still soft, the Mussel, 

 by means of its foot, applies the extremity of the filament, which 

 is dilated into a kind of little sucker, to the foreign substance 

 whereunto it wishes to adhere, and fastens it securely. Having 

 accomplished this, the foot is retracted ; and the thread, of course, 

 being drawn out of the furrow where it was secreted, is added to 

 the bundle of byssus previously existing, all of which owed its 

 origin to a similar process. 



Sometimes, instead of the numerous thin filaments met with 

 in the Mussel, the byssus consists of a single, thick, horny stem ; 

 while in other cases, as, for example, in Pinna, the threads are so 

 numerous, soft, and delicate, that they are not unfrequently spun 

 like silk, and manufactured into gloves and other small articles 



