XI MOLLUSCA 151 



Classifkalion of the Bivalve Molluscs mentioned in Chapter XL 



Family I. Unionidae (Fresh -water Mussels). — Foot long, com- 

 pressed, without byssus ; no siphons present. 

 Unio margaritifer, the Pearl MusseL 

 Anodon cygnea, the Swan Mussel. 

 ,, II. Breissenidae. — Foot cylindrical with a byssus ; two 



siphons developed. 

 Dreissena polymorpha, the Chambered Mussel or 

 the Zebra MusseL 

 „ III. Sphaeriidae (Fresh-water Cockles). 

 Sphaerium, the Orb-shell. 

 Piddium, the Pea-shell. 

 „ IV. Gardiidae (Cockles). — Much folded gills, long bent 

 foot, byssus present, short siphons. 

 Oardium edule, the Common Cockle. 

 „ V. Ostreidae (Oysters). — Shell fixed by one valve, no 



byssus, rudimentary foot. 

 Ostrea edulis, Edible Oyster. 

 „ VI. Pectinidae (Scallops). — Ribbed shell, byssus absent or 



rudimentary, mantle-edge possessing " eyes " 

 and little tentacles. 

 Pecten maximus, the Common Scallop. 

 „ VI [. Mytilidae (Sea-mussels). — Symmetrical shell, long 

 byssus. 

 Mytiliis edulis, the Sea-mussel. 



Practical Note on Bivalves 



The Fresh-water Mussel is a useful and hardy inmate of a tank 

 such as that described at the end of Chapter IX. It should 

 always be given some sand on the tank bottom, in which it can 

 bury its foot. It will then hold its body sloping obliquely up, 

 with the front end close to the ground and the hind end much 

 higher, and with the inhalent and exhalent apertures open for the 

 passage of water in and out. 



Its use in the tank consists in the current it causes in the 

 stagnant water, and also in the fact that it takes in and digests any 

 little particles of dead organic matter that may be floating in the 

 water. To provide it with plenty of fresh microscopic food, a 

 bunch of fresh weeds from a pond should sometimes be dipped 

 into the surface water and shaken. 



Sketches should be made (1) of the external view of the mussel 



