202 MALACOZOA. TROPIOPOPA. 



ligament, which throws them open, when the adductor 

 muscles are relaxed. 



The Tropiopoda so graduate into each other that the 

 attempts to separate them into orders have been unsuc- 

 cessful. Lamarck instituted a division of them into 

 Monomyaria and Dimyaria, or those with one adductor 

 muscle, and those with two, and many authors have 

 adopted this arrangement, confessing however that there 

 is no real distinction between the groups, and that species 

 considered by some as Monomyarian are in fact furnished 

 with two muscles, although the anterior muscle is very 

 small. A little more complexity in the organization of 

 a cockle than in that of a clam does not appear to me to 

 furnish a sufficient reason for referring these animals to 

 two distinct orders. It is even difficult to divide this 

 class into well-characterized families, founded on promi- 

 nent distinctions in the organization of the animals, 

 while nothing can be more easy than to form groups 

 depending merely on differences in the shells. 



Many of these animals live absolutely fixed to a par- 

 ticular spot, others have merely a little locomotion in a 

 hole in sand or mud, some are enclosed in stone or wood, 

 and few move about from place to place, the foot or 

 fleshy appendage of most of them not being formed for 

 creeping. They are all aquatic, and feed on organic 

 particles. Many species are used as food, some being 

 generally esteemed delicious, and in this respect they rank 

 much higher than the Gasteropoda. 



