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Show 110 GEOGRAPliiCAL DISTRIBUTION OP TESTACEA• (Ch. VII, to the two islands, which are only separated by l1 distance of twelve leagues; and two even of these four (namely, Helix rlwdostoma and H. ventrosa) are species of general dif~ fusion, common to Madeira, the Canaries, and the South of Europe*. The confined range of these molluscs 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 isles and the European continent. May not the eggs, when washed into the sea by the undermining of cliffs, float uninjured to a distant shore? Notwithstanding the proverbially slow motion of snails and molluscs 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 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 serpulre 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. An illustration of the mode of attachment of these eggs will be seen in the annexed diagram. (No. 1.) ~ Camb. Phil. Trans. vol. iv. 1831, Ch. VII.] DISTRIBUTION OF ZOOPHYTES. 111 Egg1 qf .frtlh·n!ale'l' MoUIJICI, Fig. 1. Eggs of Ampullaria ovnta (a fluviatile species), fixed to a small sprig which had fallen into the water. Fig. 2. Eggs ofPlanorbisn.lbus, attached to a dead leaf lying under water. Fig. 3. Eggs of the common Limneus (L. vulgaris), adhering to a dry stick under water. The habit of some testacea to adhere to floating wood is proved by their fixing themselves to the bottoms of ships. By this mode of conveyance Mytilus polymorphus has been brought from northern Europe to the Commercial Docks in the Thames, where the species is now domiciled. Geographical Distribution and Migrations of Zoophytes. Zoophytes are very imperfectly known, but there can be little doubt that each maritime region possesses species peculiar to itself. The madrepores, or lamelliferous polyparia, are found in their fullest development only ·in the tropical seas of Polynesia and the East and West Indies, and this family is represented only by a few species in our seas. Those even of the Mediterranean are inferior in size, and, for the most part, different from such as inhabit the tropics. Peron and Lesueur, |
