BIVALVE MOLLUSCS. 251 



slugs are touched by the lime they begin to give out 

 a great deal of slime, and many of them, especially 

 the younger ones, are killed and become black. Very 

 many of the older slugs, however, escape death, since 

 the tenacious slime they give out, when the lime 

 touches them, forms a continuous coat like a cocoon, 

 out of which the slug creeps, now much poorer in 

 water. It is therefore a good plan to strew powdered 

 lime over a slug-infested field twice in succession, 

 leaving an interval of some ten or fifteen minutes. 

 The second strewing brings the lime into contact with 

 those slugs which have just crawled out of their slimy 

 investment in a desiccated condition. They are unable 

 to form a second coating of slime, and if touched for 

 this second time with even the smallest particles of 

 lime they at once contract and die. If a double strew- 

 ing is repeated very early on the following morning, 

 no slugs will be left alive on the field so treated. 



CLASS: lAMELLIBRAHCHIATA rBIVALVE 



MOLLUSCS). 



Headless. The mantle consists of two flaps, which 

 secrete two shelly pieces (valves) movably united on 

 the dorsal side (Fig. 43). The foot is usually wedge- 

 shaped. Respiration effected by gills placed in the 

 mantle-cavity, which communicates with the exterior 

 either by the usual respiratory opening, or by a 

 breathing-tube (siphon). Most species are marine, 

 but some live in fresh water. No kind is of agri- 

 cultural importance. To this class belong the Oyster 

 (Oatrea edulis), the Sea Mussel (Mytilus edulis), the 

 tropical Pearl Mussel {Meleagrina margaritifera), 

 the freshwater Swan Mussel {Anodonta cygnea), the 

 notorious SMp Worm {Teredo navalis), etc. 



