The confined range of these mollusks may easily be explained, if we admit that species have only one birth-place; and the only problem to be solved would relate to the exceptions—to account for the dissemination of some species throughout several islands, and the European continent. May not the eggs, when washed into the sea by the undermining of cliffs, or blown by a storm from the land, float uninjured to a distant shore?
Their mode of diffusion.—Notwithstanding the proverbially slow motion of snails and mollusks in general, and although many aquatic species adhere constantly to the same rock for their whole lives, they are by no means destitute of provision for disseminating themselves rapidly over a wide area. "Some Mollusca," says Professor E. Forbes, "migrate in their larva state, for all of them undergo a metamorphosis either in the egg or out of the egg. The gasteropoda commence life under the form of a small spiral shell, and an animal furnished with ciliated wings, or lobes, like a pteropod, by means of which it can swim freely, and in this form can migrate with ease through the sea."[920]
We are accustomed to associate in our minds the idea of the greatest locomotive powers with the most mature and perfect state of each species of invertebrate animal, especially when they undergo a series of transformations; but in all the Mollusca the reverse is true. The young fry of the cockle, for example (Cardium), possess, when young or in the larva state, an apparatus which enables them both to swim and to be carried along easily by a marine current. (See [fig. 99].)
Tne young fry of a cockle (Cardium pygmæum,) from Loven's Kongl. Vetenskaps. Akadem. Handling, 1848.
A, The young just hatched, magnified 100 diameters. B, the same farther advanced.
a, The ciliated organ of locomotion with its filamentous appendage b.
c, The rudimentary intestine.
d, The rudimentary shell.
These small bodies here represented, which bear a considerable resemblance to the fry of the univalve, or gasteropodous shells above mentioned, are so minute at first as to be just visible to the naked eye. They begin to move about from the moment they are hatched, by means of the long cilia, a, a, placed on the edges of the locomotive disk or velum. This disk shrinks up as they increase in size, and gradually disappears, no trace of it being visible in the perfect animal.
Some species of shell-bearing Mollusca lay their eggs in a sponge-like nidus, wherein the young remain enveloped for a time after their birth; and this buoyant substance floats far and wide as readily as sea-weed. The young of other viviparous tribes are often borne along entangled in sea-weed. Sometimes they are so light, that, like grains of sand, they can be easily moved by currents. Balani and Serpulæ are sometimes found adhering to floating cocoa-nuts, and even to fragments of pumice. In rivers and lakes, on the other hand, aquatic univalves usually attach their eggs to leaves and sticks which have fallen into the water, and which are liable to be swept away during floods, from tributaries to the main streams, and from thence to all parts of the same basins. Particular species may thus migrate during one season from the head waters of the Mississippi, or any other great river, to countries bordering the sea, at the distance of many thousand miles.