

FOOT SECRETION OF BYSSUS. 497 



the water is thrown, he plunges his rod into the sand, and generally 

 succeeds in piercing the animal with the barbed extremity, and drag- 

 ging it from its concealment ; but should he fail in his first attempt, 

 he well knows that to try again would be unavailing, for the animal 

 instantly works its way down to such a distance as to render pursuit 

 hopeless. 



(1301.) But, however efficient as a means of burrowing the foot may 

 be, it can be turned to other purposes. The Pholades, for example, by 

 some means, either of a mechanical or chemical nature, not as yet pre- 

 cisely determined, excavate the solid rocks, and form therein chambers 

 in which they pass their lives. In such genera, the foot, which would 

 be useless as a boring instrument, by being simply transformed into a 

 broad and flat disk becomes a powerful sucker, whereby the Pholas 

 fixes itself to the walls of its apartment in any convenient situation. 



(1302.) In many of the Cockle-tribe we find the foot converted into 

 an instrument of locomotion of a very singular description, enabling the 

 cardiaceous Conchifera to leap by bounds we should 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 fig. 252; when thus fixed, a sudden 

 spring-like action of the muscles of the foot throws the cockle into the 

 air, and, by a repetition of these exertions, the creature can skip about 

 with surprising agility. 



(1303.) 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 vio- 

 lence 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. 



(1304.) The foot in the Mussel is of small dimensions, being useless 

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

 ment to the horny threads of the byssus, which are individually 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 secreting 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 follows : 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 



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