of Mollusca" has been given to the time during which these creatures lived, there being found in great numbers also representatives of the other classes of Mollusca, though not in so great a profusion as Brachiopods. The Brachiopoda of the present seas are restricted to a few genera, the class having nearly died out. This is true, comparatively speaking, of the rest of the Mollusca, though not in so great degree. This fact of the abundance of the Mollusca in the oldest rocks, and of the Brachiopoda in particular, harmonizes with the facts of their structure and development in showing that the group must have branched off from the main trunk (worms) very early in time, and that of the Mollusca the Brachiopoda are the oldest. In some Gasteropoda the feet appear modified as wings, as in Hyalsea and Cleodora, constituting the Pteropoda, and offering the transition to the Cephalopoda. This view is confirmed by the embryos of the Gasteropoda, Pteropoda, and Cephalopoda (Fig. 53) being so very similar. The Gasteropoda breathe both by gills and lungs; examples of the gill-breathing kind are seen in the Whelks (Buccinum) (Fig. 52) often picked up on the sea-shore while the garden-snail will represent the lung-breathing kind. The beautiful Carinaria, with its delicate propeller, is a highly specialized gill-breather. The Cephalopoda are the most highly organized of the Mollusca. The Cuttle-fishes (Fig. 54), with their long arms, are familiar to all who have read the "Toilers of the Sea." In them we find the nervous system well developed, eyes are present, the viscera are large, while blood circulates through arteries and veins. The Cuttle-fish is able to conceal itself through emitting a very brownish-black fluid, which is contained in the so-called ink-bag. They breathe by two gills, but in one genus, the Pearly Nautilus, four gills are present. The Nautilus is the only living representative of myriads of fossil forms, the Ammonites, and is probably the
ancestor of the two-gilled Cephalopods. The evidences of
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