. FIG. 13. appearance, its position towards the testa, head, mouth, and foot, and its subsequent effacement, one and all coincide exactly in the two classes. It is as yet of only a relatively small number of marine shells and slugs that we know the evolutionary history; yet we may infer that in these animals remaining in their orii:;;inal home, this heirloom has been generalh' preserved. Even genera which in their mature state scarcely recall the type of the iNIollusca, as the boring mollusks (Dentalium Teredo), have preserved the phase of the navicula. On the other hand, in the branchiate fre
. FIG. 13. appearance, its position towards the testa, head, mouth, and foot, and its subsequent effacement, one and all coincide exactly in the two classes. It is as yet of only a relatively small number of marine shells and slugs that we know the evolutionary history; yet we may infer that in these animals remaining in their orii:;;inal home, this heirloom has been generalh' preserved. Even genera which in their mature state scarcely recall the type of the iNIollusca, as the boring mollusks (Dentalium Teredo), have preserved the phase of the navicula. On the other hand, in the branchiate fresh-water snails (Paludina) the velum is little developed, and in the land snails, which differ most widely from their marine kindred, the velum is entirely obliterated, as it is also among fresh- water mussels. If in these animals adaptation and migration to land has had this effect on embr\onic and post-embryonic development, we must suppose that in
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Keywords: ., bookauthorschmidtd, bookcentury1800, bookdecade1890, bookyear1896
