MOLLUSCA. 263 
front pair being the true tentacles, and the long hinder 
pair being the eye-stumps. They have a saw-like upper 
Fie. 223.— A, Land-snail (Helix); B, C, D, Slugs (Limax); E, F, G, Pond-snails 
(Limneea, Paludina, and Planorbis). 
jaw for biting leaves, and a short tongue covered with 
minute teeth. The Pond-snails, as Limneea and /lanor- 
bis, differ in having no eye-stalks, the 
eyes being at the base of the tentacles. 
They are obliged to come frequently to 
the surface of the water to breathe. 
4. Prosobranchs.—These are aquatic 
Gasteropods, breathing by gills situated 
in front of the heart. They are the 
most highly organized and the most 
abundant of the crawling Mollusks. 
Nearly all are marine, and all have a 
Fria. 224.—Bulimus oblon- 
shell. gus; one-half natural 
Among the lower forms are the sin- size. Guiana. 
gular Chiton, covered with eight shelly plates; Limpet 
(Patella), well known to every sea-side visitor; and the 
