The doctrine of descent and Darwinism . constituting two natural classes, havea common larval form, or, if the larvae display differentshapes, a highly distinctive common larval organ, thevelum. The accompanying diagram gives on the rightthe navicula of a cockle-shell as seen from behind. Atthe anterior end, two fleshy lobes have been formed, edgedwith cilia, by the vibrations of which the young animal,even in the egg, performs spiral twisting motions ; inthe midst of the cilia rises a little prominence, furnishedwith a longer filament. These ciliated lobes or vela,merging into one another, ar
The doctrine of descent and Darwinism . constituting two natural classes, havea common larval form, or, if the larvae display differentshapes, a highly distinctive common larval organ, thevelum. The accompanying diagram gives on the rightthe navicula of a cockle-shell as seen from behind. Atthe anterior end, two fleshy lobes have been formed, edgedwith cilia, by the vibrations of which the young animal,even in the egg, performs spiral twisting motions ; inthe midst of the cilia rises a little prominence, furnishedwith a longer filament. These ciliated lobes or vela,merging into one another, are shown on the left in thelarva of a sea-snail (Pterotrachea), as seen nearly inprofile, and in the phase in which the eyes and auditoryapparatus, the foot and operculum, as well as a delicate 200 THE DOCTRINE OF DESCENT. shell, have made their appearance. Here also, from theplane of the velum, a small fleshy protuberance juts out,without any special purport. The distribution of thevelum, the period at which this larval organ makes its. Fig. 13. appearance, its position towards the testa, head, mouth,and foot, and its subsequent effacement, one and allcoincide exactly in the two classes. It is as yet of onlya relatively small number of marine shells and slugsthat we know the evolutionary history; yet we may inferthat in these animals remaining in their original home,this heirloom has been generally preserved. Even generawhich in their mature state scarcely recall the type ofthe Mollusca, as the boring mollusks (Dentalium Teredo),have preserved the phase of the navicula. On the otherhand, 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, thevelum is entirely obliterated, as it is also among fresh-water mussels. If in these animals adaptation andmigration to land has had this effect on embryonic andpost-embryonic development, we must suppose that in OTHER FORMS OF DEVELOPMENT.
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Keywords: ., bookauthorschmidtd, bookcentury1800, bookdecade1880, bookyear1882
