100 ANIMAL STUDIES 



Others breathe by means of gills while under water, and by 

 the surface of the body and the moist walls of the mantle 



FIG. 56. 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. 



95. 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 



