PHYLUM MOLLUSCA 217 



show, at the present time, all stages of transition from ocean, 

 to fresh water, and to land. Most marine snails breathe 

 directly from the water through feather-like gills, and have 

 a free swimming larva (veliger). The fresh- water species 

 do not have free swimming larvae and, though many 

 breathe like their marine relatives, some, like Physa, have 

 rudimentary gills and respire air through the walls of the 

 mantle cavity. Succinea is a common swamp snail which 

 is amphibious. It breathes air, but may live in the water 

 or on land, and is as often found perched on the top of a 

 cat-tail as among the plants under water. Land snails lay 

 their eggs in holes in the ground and in rotten logs; they all 

 breathe air into the mantle-cavity and are without gills. 



Several of the large marine snails are of considerable 

 economic importance because they are pests to oyster and 

 clam fishermen. They bore holes in the shells of other 

 molluscs with their radulas and extract the soft parts. 

 The slugs, which are snails with very rudimentary shells, 

 and other land snails often do damage to gardens by gnaw- 

 ing holes in the vegetables. The large snail in Europe, 

 Helix pomatia, is a source of trouble in vineyards and fields, 

 but is also of value as a food. It is raised in " snail farms"* 

 and gathered in the fields for local consumption and export. 

 Most of the edible snails used in the United States are sold 

 to the restaurants in the large cities. The Department of 

 Agriculture has recently issued a bulletin describing meth- 

 ods of culture and suggesting that snail farming be under- 

 taken in this country. 



CLASS 2. LAMELLIBRANCHIATA 



Here belong the clams, mussels, oysters, scallops, ship 

 worms, and other " bivalves. " These animals obtain their 

 food by siphoning large quantities of water through the 

 mantle cavity and retaining the minute particles. Cur- 

 rents created by the cilia covering the body and mantle 

 bring in small plants and animals which are swept into the 



