Those genera of the Trachelipods which live on vegetable substances
have no projecting siphon, but possess a mouth furnished with jaws;
they are in general land-shells, and consequently, the air which
they breathe is conveyed directly to their branchiæ. Some of their
tribes however, live in fresh water, either in running streams or in
stagnant pools: among these, some breathe water and others air. These
last are obliged frequently to come to the surface for the purpose of
breathing,—others again inhabit salt water, and are unable to exist
out of that element.