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.)

[Illustration: Fig. 99.

Tne young fry of a cockle (Cardium pygmaeum,) 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 Serpulae 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 cut. (Fig. 100.)

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_, previously known only in the Danube and Wolga,
may have been brought to the Commercial Docks in the Thames, and to
Hamburgh, where