VI INTRODUCTION. 



nature has provided them, for the purpose of fixing 

 their shells to rocks, or stones. The foot in these 

 animals is always smaller, more pointed, and chan- 

 nelled by a longitudinal groove, intended to give 

 shape to the thread, which first exudes, like gum, 

 from the end of it. The Muscle and the Pinna are 

 examples of this division. They are capable of loco- 

 motion ; but unless torn from their hold, by a supe- 

 rior force, they generally remain in the place where 

 they first established themselves. 



The AcSphales without siphons, which have no 

 foot, are fixed for their lives, by a calcareous ce- 

 ment. These, such as oysters, are of the lowest order 

 of animated beings ; their organization is extremely 

 simple ; they have merely two holes for the mouth, 

 &c. four pulmonary leaflets, called the beard, and a 

 mantle. 



All the animals of Bivalves are fixed to their 

 shells by a muscle, which is attached to the hinge, 

 and by one, two, or more, fixed to the interior of 

 the valves, in the places which conchologists call 

 muscular impressions. These last muscles are prin- 

 cipally designed to close the sides of the shell ; that 

 they do it effectually, all will acknowledge, who are 

 used to the opening of oysters. The force with 

 which they close their shells varies, of course, in 

 proportion to the strength of their muscles. Poli, 

 who tried several experiments on the muscular 



