Reference has already been made to the labours 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 appreciable 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 considerably from the adult. That of _Anodon_, the fresh-water mussel, at first received, in consequence, 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 exceptional in the fact that it is parasitic on fish. The usual Molluscan larva is a ciliated creature 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 because 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 development is necessarily much more direct. It is worthy of note that the periwinkle mentioned above, which lives high and dry (_L. rudis_) has no larval form, while its relatives that live under water develop in the usual way. The eggs of Mollusca are often enclosed in tough cases, calculated to resist waves and weather. Some of these are shown in miniature, in the group of eggs of various kinds, Fig. 35. [Illustration: FIG. 35.--Eggs (reduced to half the natural size). _A_, Egg-Capsules of _Murex_. _B_, Frog's Eggs. _C_, Eggs of large Land-snail. _D_, Eggs of Snail placed on a leaf. _E_, Cockchafer's Eggs. _F_, Egg-case of Cockroach. _G_, Egg-cases of Locust. _H_, _I_, _J_, Eggs of Gasteropod Molluscs. _H_, _Sycotypus_ (_Pyrula_). _J_, _Fusus_.] TABLE SHOWING THE CLASSIFICATION OF THE MOLLUSCA { AGLOSSA: the LAMELLIBRANCHIATA, also called { CONCHIFERA, and PELECYPODA. =MOLLUSCA.= { { { GLOSSOPHORA { GASTEROPODA. { CEPHALOPODA. CHAPTER X THE BRACHIOPODA OR LAMP-SHELLS These were at one time included under the Mollusca, on account of their possession of a bivalve shell. This shell, however, is placed practically back and front of the animal, not to the right and left of it, as is the case with the shells of the bivalve Mollusca.