A LAND SNAIL AND OTHER MOLLUSKS 161 



snail's food are rasped from the plants by a thin, filelike band 

 covered with minute backwardly pointed teeth. This organ, 

 the radula (Fig. 90, 4), is protruded from the mouth and drawn 

 across the plant, thus scraping off very fine particles. A sort of 

 jaw is also present (Fig. 90, j) which aids the radula by cutting 

 off pieces of the plant for the radula to work upon. 



Respiration. The terrestrial habit of the snail requires an 

 entirely different breathing apparatus from that of its aquatic 

 relatives, and instead of a mantle cavity filled with water its 

 mantle cavity has become a sort of lung (Fig. 89, Mh.}. Air is 



FIG. 91. Flashlight photograph of earthworm and slug crawling on a pave- 

 ment at night. (From Davenport.) 



taken into and expelled from this cavity, and the exchange of 

 oxygen and carbon dioxide takes place between the air inhaled 

 and the blood in the numerous blood vessels that are present in 

 the lining of the mantle cavity. 



Slugs. Besides the ordinary land snails there are a few 

 terrestrial gastropods that are so peculiar as to deserve special 

 mention; these are the slugs (Fig. 91). Slugs may be found 

 under boards or stones in damp places. They are apparently 

 without a shell, but there is a thin, horny plate embedded in the 

 mantle which is the last remnant of what was no doubt in the 

 slug's ancestors a fully developed shell. Some slugs, especially 

 the introduced species, Limax maximus, are a nuisance in green- 

 houses because of their attacks on plants. 



