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 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 



