2 3 o ECONOMIC ZOOLOGY AND ENTOMOLOGY 



worm-like, and indeed are more commonly thought to be worms 

 than bivalve molluscs. They bore into the wood only for 

 protection, and do not feed upon it. Their food consists of 

 minute organisms that are taken into the body through the 

 siphons. 



Teredos are very serious pests on the piles of wharves, and 

 on dykes, ships' bottoms and any other wood that comes in 

 contact with salt water. In some places they are so abundant 

 that a two-inch plank may be completely honey-combed and 

 destroyed in less than a year. The only protection is to cover 

 the wood with some substance which the teredo cannot or will 

 not penetrate. Heavy coatings of copper or verdigris paint 

 are often used, but they must be reapplied frequently. Cer- 

 tain other Pelecypod molluscs have the remarkable habit of 

 boring into solid rocks far enough to protect them. 



CLASS GASTROPODA 



Snails. Snails are very common objects in water and on 

 land. They all have shells, which may be conical or spire- 

 shaped or flattened. The most common snails have spiral, 

 more or less cone-shaped shells. One group, the pulmonate 

 snails, including many common aquatic and terrestrial forms, 

 do not breathe by means of gills as do most other molluscs. 

 On the right side of the body near the anterior end is an exter- 

 nal opening that leads into a sac, the so-called "lung." The 

 inner surface of this sac is abundantly supplied with fine blood- 

 vessels through the walls of which oxygen is taken from the 

 air and carbon dioxide thrown off. These snails are vege- 

 table feeders and are sometimes serious pests among flowers 

 and in the garden. 



The members of another group of common pond snails 

 have gills and no lung-sac. These live on the bottom of the 

 ponds and feed on animal rather than vegetable food. 



Most of the snails and the slugs have two pairs of " horns 1 ' 

 with the eyes on the tips of the second pair. Some snails have 

 only one pair, which are used as feelers, the eyes being situated 

 at the base of these feelers. 



