Shell-fish industries . similarities in develop-ment, structure, and habit, which exist among oysters,clams, scallops, mussels, and other members of the lamelli-branch family, no naturalist now doubts that theydescended from some common ancestor, which, however,must have lived in the very remote past, as man meas-ures time. What this ancestral form was, is not posi-tively known; but naturalists have agreed on what musthave been the general characters of many of its they should have a positive belief in regard to acreature that no one ever saw, even in fossil form, is along story; bu
Shell-fish industries . similarities in develop-ment, structure, and habit, which exist among oysters,clams, scallops, mussels, and other members of the lamelli-branch family, no naturalist now doubts that theydescended from some common ancestor, which, however,must have lived in the very remote past, as man meas-ures time. What this ancestral form was, is not posi-tively known; but naturalists have agreed on what musthave been the general characters of many of its they should have a positive belief in regard to acreature that no one ever saw, even in fossil form, is along story; but the reasons for it, if they were explained,would probably be satisfactory to most minds. Among the very few bivalves here considered, it isnot easy to determine which, in its structure, conformsmost closely to the hypothetical ancestor. It is not theblack mussel, with its aborted foot and anterior adductormuscle, and its sexual glands in the mantle folds. It isnot the scallop, in which much of the body is modified to. Anatomy of the Food Mollusks 13 conform to the swimming habit. Certainly it is not thedegenerate oyster that has completely lost the organ oflocomotion, and the anterior adductor muscle. Probablyit is not the soft clam, for in it, also, the ancient foot isgreatly reduced. Of the short list, the hard clam, Venusmercenaria, probably has a greater number of organs thatare most like those of the ancestral bivalve, though some,like the gills, depart much farther from the primitive con-dition of those organs than do those of the mussel andscallop. But because Venus, not by any means one of themore primitive of living bivalves, is somewhat the moresimple of the species here described, it may illustrate bestsome of the anatomical characters common to them all. The Shell. The hard protective covering of Venusconsists of right and left parts known as valves. It iscomposed of carbonate of lime, which is deposited in aviscous secretion poured out by the fleshy mantle foldli
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