440 TEXT-BOOK OF ZOOLOGY 



ness, the animal lies as in a dish, the lid of which is formed by the 

 shallow and thinner upper valve. In the motionless oyster there is 

 little difference between anterior and posterior ends; the current of 

 water flows in along the whole ventral edge of the Bhell, and the foot, 

 which has become useless, completely atrophies. These conditions 

 make it easy to understand that oyster colonies — "oyster banks," as 

 they are called— can only be formed on hard ground, washed over by 

 tidal currents, which convey to the sessile animals their necessary food. 

 In Germany the deep channels on the west coast of Schleswig-Holstein 

 are the only situations adapted for the habitat of the oyster. In the 

 Baltic it is entirely absent. The oyster fishery is carried on by means 

 of dredges. 



Another . edible bivalve, the Sea Mussel {Mytilus edulis), attaches 

 itself in a different manner. This animal, like many other bivalves 

 (e.g., the pearl oyster ; see above), possesses in its foot a gland which 

 secretes a viscid fluid. When this comes in contact with water it at 

 once hardens into threads (byssus), by which the animal attaches itself 

 to the supporting surface. Thousands of these beautiful blue-shelled 

 bivalves may often be seen attached to the piles of piers, etc. On the 

 coast of France mussels are cultivated on stakes and hurdles fixed in the 

 mud. 



Another species of similar form, the triangular Dreissena polymorjpha, 

 which also attaches itself by a bysgus gland, has been introduced by 

 ships and rafts from the Black Sea into a very large number of European 

 rivers. 



Another species, the Cockle (Cardium edule), is much consumed in 

 Southern Europe, Britain, and Holland. This mollusc is found on all 

 the European coasts, and sometimes in such quantities that the shells 

 are burnt to make lime, or are used as road-metal. The animal is 

 fond of completely burying itself in sand or mud. As it is necessary, 

 however, that it should remain in communication with the water (why?), 

 the edges of the mantle coalesce at the afferent and efferent orifices, 

 and form tubes which project above the sand or mud into the water. The 

 animal, however, remains to some extent freely movable, and by means 

 of its foot is capable of moving forwards by jumping. Hence the 

 siphonal tubes remain short, whereas in those species which are per- 

 manently sessile in the mud they often attain to a considerable length 

 and become concrescent. This is seen in the Boring-Shells (Pholas}, 

 which burrow deep down into wood, and even into solid rock. This they 

 accomplish by means of their foot and shell, the anterior end of which is 

 rough, like a file. Their elongated Bhape assists the animals in penetra- 

 ting these objects. They are, however, unable to leave their self-created 



