200 THE DOCTRINE OF DESCENT. 
shell, have made their appearance. Here also, from the 
plane of the velum, a small fleshy protuberance juts out,. 
without any special purport. The distribution of the 
velum, 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 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 original home, 
this heirloom has been generally preserved. Even genera 
which in their mature state scarcely recall the type of 
the Mollusca, 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 embryonic and 
post-embryonic development, we must suppose that in 
