114 THE STORY OF ANIMAL LIFE 



belong to the Glossophora or Tongue-ribbon 

 Carriers, of which they present a comparatively 

 primitive form. 



Reference has already been made to the la- 

 bours of the earthworm and of the insects, and to 

 their important effects upon the vegetable world. 

 Although the Mollusca include but one terrestrial 

 group, the Snails, they, too, have played an ap- 

 preciable part in modifying plant life. If we owe 

 our flowers to the insects, we have probably to 

 thank the snail for our medicines. For the snail 

 dislikes bitter-tasting leaves, and lets them alone, 

 thus exercising an artificial selection in favour 

 of the survival of medicinal plants. In the same 

 way the snail has favoured the survival of hairy 

 and thorny plants, upon which it cannot easily 

 crawl. 



The larval forms of the Mollusca differ con- 

 siderably from the adult. That of Anodon, the 

 fresh-water mussel, at first received, in conse- 

 quence, a different name, that of Glochidium, by 

 which it is still known, although it has now been 

 long identified as a larval form. It is excep- 

 tional in the fact that it is parasitic on fish. 



The usual Molluscan larva is a ciliated crea- 

 ture which has been compared to a modified 

 trochosphere. It is preceded by a gastrula stage, 

 and it develops later on into what is called a 

 " Veliger," or "veil-carrying" larva, so called be- 

 cause it has in front a broad two-lobed ciliated 

 expansion, the velum. This larva is adapted for 

 swimming, which is accomplished by means of 

 the velum. In terrestrial molluscs, the devel- 

 opment is necessarily much more direct. It is 

 worthy of note that the periwinkle mentioned 

 above, which lives high and dry (L. rudis) has 



