168 



A COURSE ON ZOOLOGY. 



FlQ. 97. 



all are aquatic, some living in fresh, others in salt water. 

 In some the respiratory organs are analogous to lungs ; 

 in others they are gills, which are 

 situated sometimes internally, some- 

 times externally. 



Among the gasteropods having 

 lungs, the more important are the 

 edible snails or helixes, the slugs or 

 naked snails, and the various pond 

 snails. Of the latter, two very small 

 species, the limnsea and the planorbis, 

 abound in stagnant waters ; the former 

 have conical shells, while those of the 

 latter are discoid. Edible snails are 

 found in all quarters of the globe ; 

 they live on leaves and fruits, and, 

 although their mouth has but a single 

 tooth, they can do great damage in 

 vegetable gardens. Their use as food 

 was for a long time restricted to very 

 limited districts, but they are now much sought for 

 the table. The most highly- esteemed species is the 

 vineyard snail, having a russet color, with paler stripes, 

 and 'commonly found in vineyards. The slugs having 

 a viscous skin and u repulsive appearance are quite as 

 destructive as the edible snails. The most common 

 species are the red snail, found in the woods, the cellar 

 snail, and the black snail. 



The gasteropods having gills can only be mentioned for 

 the varied and often very singular appearance of their 

 shells. Among them may be named the top, the bubble, 

 pouch, boatman, hatchet, dolphin, all named from their 

 curious forms, and the various limpets. Under the name 



ac 



SECTION OF TRITON- 

 SHELL (after Owen). 

 ac, notch for si- 

 phon ; c, axis or col- 

 umella. 



