32 MOLLUSCA. 
species of the same genera are never or rarely seen 
in gardens, but devour the herbage of the roadside, 
the bank, or the hedge. Many, particularly those 
which inhabit the woods of foreign countries, de- 
vour the leaves of trees. The plant-eaters among 
the marine tribes live upon the various kinds of 
sea-weeds, of which there is a sufficient variety to 
gratify a taste much more epicurean than it pro- 
bably is in reality. The common Periwinkle and 
the Limpet are both vegetable feeders, and there is 
a pretty little species of the latter genus which 
invariably, I believe, confines itself to one plant: 
this is the Patella pellucida, distinguished by having 
on its summit three or four lines of blue, most 
brilliantly gemmeous. It feeds on the tangle, 
(Laminaria digitata) eating away a cavity for 
itself, just large enough to contain its body, in the 
substance of the cartilaginous stem, commonly be- 
neath the shelter of the arching roots. [have pulled 
up the tangles by dozens at low spring-tide, and 
have scarcely ever found one that had attained 
certain dimensions without finding a little parasitical 
Limpet embedded in its substance. 
If we measure the interest which we take in any 
section of created beings by their powers of con- 
ferring benefit or inflicting injury on our own race, 
we shall find the Mollusca not unworthy of our 
regard in both these respects. Many of them are 
used as human food, and that not by savage nations 
only, but by ourselves and by all classes of society. 
The Limpet, the Periwinkle, the Whelk, the 
Mussel, and the Cockle, are commonly sold in the 
streets of our sea-port towns and large cities, though 
these are certainly more prized by the lower classes 
of society than by those of more cultivated tastes. 
