BIVALVE MOLLUSCA. 343 
peculiarities in the organisation of these molluscs is in direct con- 
nection with their mode of life. For those molluscs, which most 
commonly live on the sea-shore, and bury themselves in the sand to 
the depth of four or five inches, are enabled to breathe, to draw 
water for their nourishment, and also to throw off the products of 
digestion, by having the mantle prolonged, as we have seen, into two 
tubes, the orifices of which reach to the surface of the mud. By means 
of their foot, which is an extremely curious organ of locomotion, the 
Cockles can at will issue from their holes and re-enter them. The 
fishermen of the shore easily recognise the presence of these animals 
by the little jets of water which they throw up through the sands. 
Fig. 145.—Cardium aculeatum (Linnzeus). Fig. 146.—Cardium edulis (Linnzus). 
These molluscs are found in every sea on the globe, and under 
all latitudes. Many of them belong to our own and the French 
coasts, where they are eagerly sought for by collectors, as well as for 
food. The flesh of the animal, however, is somewhat leathery, and 
little esteemed. The species most common on the littoral of the 
Atlantic is Cardium edulis (Fig. 146), its white or fawn-coloured shell 
being hollowed out into six-and-twenty furrows, forming so many 
corrugated ripples on its side. It is considered good for food. 
‘The common cockle frequents sandy bays, near low water. It 
is sometimes met with in brackish water, as at the mouth of the 
Thames. 
_ Cardium costatum (Fig. 147) is an exotic species which inhabits 
the coast of Guinea and the Senegal, the shell of which, white and 
fragile, is much sought after by collectors. 
