BIVALVE MOLLUSC A. 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 (Linnaeus). Fig. 146. Cardium edulis (Linnaeus). 



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. 



