MOLLUSKS 83 
Others breathe by means of gills while under water, and by 
the surface of the body and the moist walls of the mantle 
Fic, 50.—Three different species of naked marine snails or Nudibranchs. Natural 
size, from life. 
cavity when exposed. In some of the small Littorinas 
attached so far from the sea as to be only occasionally 
washed by the surf this latter method may prevail for days 
together—in fact they live better out of water than in it. 
It is not difficult to imagine that such forms, keeping in 
moist places, might wander far from the sea, and, losing 
their gills, become adapted to a terrestrial life. It is 
believed that in past times this has actually occurred, and 
that our land forms trace their descent from aquatic ances- 
tors. To-day they breathe by a lung—that is, they take 
oxygen through the walls of the mantle cavity, as the slug 
may be seen to do, though in some species traces of the old 
gill yet remain. 
84. Food and digestive system. Many mollusks live upon 
seaweeds, and the greater number of terrestrial forms are 
fond of garden vegetables or certain kinds of lichens, but, 
on the other hand, the latter, together with a large number 
of marine snails, are carnivorous. In all cases the food 
requires to be masticated, and, unlike the clams, the mouth 
is usually provided with horny jaws, and an additional 
