The chief points in which they vary are - (1) in the structure of the ctenidia or branchial plates; (2) in the presence of one or of two chief muscles, the fibres of which run across the animal's body from one valve of the shell to the other (adductors); (3) in the greater or less elaboration of the posterior portion of the mantle-skirt so as to form a pair of tubes, by one of which water is introduced into the sub-pallial chamber, whilst by the other it is expelled; (4) in the perfect or deficient symmetry of the two valves of the shell and the connected soft parts, as compared with one another; (5) in the development of the foot as a disk-like crawling organ (Arca, Nucula, Pectunculus, Trigonia, Lepton, Galeomma), as a simple plough-like or tongueshaped organ (Unionidae, &c.), as a re-curved saltatory organ (Cardium, &c.), as a long burrowing cylinder (Solenidae, &c.), or its partial (Mytilacea) or even complete abortion (Ostraeacea).
In Unionidae and several other forms the pericardial glands are extended into diverti cula of the pericardium which penetrate the mantle and constitute the organ of Heber.
Mutelidae.-Differs from Unionidae in having two pallial sutures; freshwater.
Musculus, diminutive of mus, mouse, applied to small sea fish and mussels), a term applied in England to two families of Lamellibranch Molluscs - the marine Mytilacea, of which the edible mussel, Mytilus edulis, is the representative; and the fresh-water Unionidae, of which the river mussel, Unio pictorum, and the swan mussel, Anodonta cygnea, are the common British examples.
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