HEADLESS MOLLUSCS. 285 



will probably assist our reader in placing in its 

 corresponding class those specimens which he 

 discovers. 



Of the three subdivisions of the headless mol- 

 luscs, we shall omit the two first, the specimens 

 of which are comparatively rare. The third 

 class, however, contains a large number of fa- 

 miliar " shell-fish," and merits careful attention. 

 It is known by the sesquipedalian title of La- 

 mellibranchiata, which signifies that the gills are 

 in the form of flat plates lamella being the 

 Latin word for a plate. 



This subdivision comprehends a great many 

 well-known bivalves, or animals with shells, having 

 two separate sides attached together by hinges. 

 Of these, the cockle, the mussel, the scallop, and 

 the oyster, are the most familiar types. 



Let us suppose it to be low-water on one of 

 those sandy shores where the common cockle 

 may be found. It is a spring-tide, and the waves 

 have retreated far below their usual limit, leaving 

 a wide extent of sand quite bare. Furnished 

 with a trowel, we dig a little way below the sur- 

 face, and our labour is at once rewarded with a 

 handful of cockles. One of these will serve to 

 illustrate the internal structure of several other 

 species of bivalve molluscs. 



The shelly covering of the animal consists, we 

 perceive, of two pieces. In the cockle, the clam, 

 and some others, each of these two pieces is 

 almost hemispherical ; in the oyster, the scallop, 

 and others, they are, on the contrary, almost flat. 



