ii4 LIFE AND HER CHILDREN. 
Then we can trace these headless mollusca from 
their ocean-home gradually up into the fresh water, 
some forms living in the brackish water at the river's 
mouth, others like the fresh-water mussel buried in 
the mud of rivers ; and these do not spin threads, 
since they have no rude waves to meet, but put out 
two short siphons to the pure water above. All 
kinds of different forms with their habits we may 
study on the coasts and in the ponds and rivers ; 
but we never find a bivalve either on the land, or 
sailing in the open ocean. 
Fig. 43- 
Molluscs with heads. Vegetable-feeders. 
L, Limpet * walking, and attached. P, Periwinkle t walking, and 
closed. /, Foot ; o % operculum ; s, snout ; g, place where gills lie 
under the shell. 
These regions they are obliged to leave to the 
more highly-gifted mollusca with heads ; and when 
we have examined the little periwinkle grazing on 
the seaweed among the rocks, we shall, I think, be 
able to imagine how it was possible for some of his 
* Patella imlgaris. t Littorina littorea. 
