OF INVERTEBRATE ANIMALS. 
407 
many persons when walking along the roads, on a fine summer’s evening, after 
the fall of a refreshing shower of rain, will have observed, amongst the numerous 
species of Snails and Slugs that then appear slowly gliding over the moist foliage, 
a very large Slug (Limosella), variegated like Tortoise-shell. In the back of 
this Slug there is a shell, a very simple one, but it may be looked upon as the 
commencing or vanishing point of shells amongst these animals. If we now go 
to some lake or pond, and pluck a leaf of the Water-lily, it is a great chance but 
we find upon it the fresh-water Limpet (Patella). This animal will supply us 
with a shell almost flat, but still a little concave. In some other forms of 
Limpets we shall find the shell more concave, and presenting the form of a cone 
with a very enlarged base. From this we may trace shells which gradually 
present more acute cones, as in the Bonnet and Chambered Limpets ( Patella 
nereitoidea and P. Chinensis). A curve now begins to take place, and is well 
seen in the Tooth-shell (Dentalium). In some of the Worm-shells (Serpulce) we 
have the commencement of the spire. The spire may be next seen in a simple 
form in the Sea-ear ( Haliotis ). It becomes more developed in the Nereides 
( Nereidce ). In the Snails and Periwinkles it is still more developed, and when 
we have arrived at these forms we may yet trace its complication through the 
Common Whelks ( Buccinum ) to the elegant forms of the Dippers (j Bulled) and 
Cowries ( Cyprcea ). 
The shells of the Univalve Mollusca are many of them very highly prized; 
there is one in a museum at Paris belonging to the Cone tribe, which is not more 
than two inches long, and has been valued at three hundred guineas; many 
others exist, valued at from twenty to two hundred guineas. In some parts of 
the world they are employed as a medium of circulation, instead of coin. The 
Cowries are mostly used for this purpose. But many of them serve for more 
useful purposes. In France a large species of Snail ( Helix pomatia ) forms a 
delicious article of diet; whilst the Periwinkle and the Whelk are esteemed in 
our own country. 
The Bivalves, of which we have many familiar examples, as in the Oyster, the 
Cockle, the Muscle, &c., may be divided into two sections; first, those which 
bore or live in rocks, wood, mud, &c., and those which live free in water, and 
are enabled to walk about by the aid of a foot, which projects from between 
their shells. 
Among those which bore we find the Ship-worm (Teredo navalis). These 
animals are supplied with two little shells, which, acting like rasps, enable it to 
penetrate wood. They attack wood wherever it is found in the ocean, and seem 
to occupy it more as a resting place than for any other purpose. 
Another kind of these Bivalves attack even a harder material than wood, and 
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