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 efifacement, one and all co- 

 incide 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 gen- 

 era which in their mature state scarcely recall the type of 

 the Mollusca, as the boring mollusks (Dentalium Tere- 

 do), 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 



