283 



CHAP. XX. 



MOLLUSCA, OR SOFT-BODIED ANIMALS BIVALVES. 



Molluscs. Bivalves. Structure and] Habits of various 

 Species. The common Cockle, &c. 



IN those districts of our sea-shores where the 

 beach is covered with sand, or fine gravel, a very 

 considerable variety of empty shells may often be 

 found strewed along the limit of high-water. 

 Many of these are bivalves, such as the common 

 mussel, and many are univalves, like the peri- 

 winckle. All these shells, with the exception of 

 those of crabs, which sometimes are cast ashore 

 with the others, belong to a very extensive pri- 

 mary group of animals, called molluscs, from the 

 Latin word signifying soft, a quality which dis- 

 tinguishes them from the jointed or articulated 

 animals we have been considering. 



The group of animals to which we are now to 

 give our attention, exhibit great diversity, not 

 only as respects size and shape, but as regards 

 the places they inhabit. 



Some of the shells of microscopic molluscs are 

 so inconceivably minute as to pass readily through 

 a hole pierced in paper with the point of a fine 

 needle. Others, again, are of enormous dimen- 

 sions; the Giant Clamp-shell, a huge bivalve, 



