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].
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
| MOLLUSCA. |  | AGLOSSA: the Lamellibranchiata, also called Conchifera, and Pelecypoda. | ||
| GLOSSOPHORA |  | Gasteropoda. Cephalopoda. | ||