the species is now domiciled. But Mr. Gray suggests that
as the animal is known to have the faculty of living for a very long
time out of water, it is more probable that it was brought in Russian
timber, than borne uninjured through the salt water at the bottom of a
vessel.[921]

A lobster (_Astacus marinus_) was lately taken alive covered with living
mussels (_Mytilus edulis_)[922]; and a large female crab (_Cancer
pagurus_), covered with oysters, and bearing also _Anomia ephippium_,
and Actiniae, was taken in April, 1832, off the English coast. The
oysters, seven in number, include individuals of six years' growth, and
the two largest are four inches long and three inches and a half broad.
Both the crab and the oysters were seen alive by Mr. Robert Brown.[923]

[Illustration: Fig. 100.

Eggs of Freshwater mollusks.

Fig. 1. Eggs of _Ampullaria ovata_ (a fluviatile species) fixed to
a small sprig which had fallen into the water.
Fig. 2. Eggs of _Planorbis albus_, attached to a dead leaf lying
under water.
Fig. 3. Eggs of the common Limneus (_L. vulgaris_), adhering to
a dead stick under water.

]

From this example we learn the manner in which oysters may be diffused
over every part of the sea where the crab wanders; and if they are at
length carried to a spot where there is nothing but fine mud, the
foundation of a new oyster-bank may be laid on the death of the crab. In
this instance the oysters survived the crab many days, and were killed
at last only by long exposure to the air.

_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. The zoophytes of the Mediterranean, according to Ehrenberg, differ
almost entirely from those of the Red Sea, although only seventy miles
distant. Out of 120 species of Anthozoa, only two are common to both
seas.[924] Paron and Lesueur, after studying the Holothuriae, Medusae, and
other congeners of delicate and changeable forms, came to the conclusion
that each kind has its place of residence determined by the temperature
necessary to support its existence. Thus, for example, they f