THE STALK-EYED CRUSTACEA, 253
Pisa tetraodon occurs at the Nore and may be considered an
Essex species.
Another interesting family are the Swimming Crabs or
Portunidae. They have the fifth pair of legs terminating in
paddles, and although they do not really "swim" in the proper
sense of the word, they possess the power of raising themselves
from the sea bottom, and making short aquatic flights. An
exception to this, however, is presented in the case of Polybius
henslowii, which has an exceedingly thin chitinous carapace.
This species has often been taken in herring-nets, a fact which
proves that it must be a hardy swimmer, as its structure would
lead one to expect. An exceedingly delicate member of this
family is Portumnus latipes, a rather rare species near our coasts,
but much more abundant on the French side of the Channel.
Portumnus puber, the "Lady Crab" of the Channel Islands, is
a handsomely marked species : unhappily the bright colours
disappear at death, so that no idea of its beauty can be formed
from a dried specimen.
The Common Shore-Crab (Carcinus maenas) is very widely
distributed and is a well-known Essex form, where its colour is
invariably identical with the sandy or muddy bottom on which
the creature lives. When, however, we find it in rock-pools on
the granite or serpentine of Cornwall, or on the syenite or
basaltic shores of the Channel Islands, this interesting species
assumes the most beautiful markings, tinted with really gorgeous
colour, and mottled in the most delicate manner. It is often
almost impossible to detect the Crab in these localities, so per-
fectly does it resemble its surroundings.
The genus Xantho (X. florida and X. rivulosa) has its carapace
very solid and strongly built up. It is found in the wildest rocky
shores of the Channel Islands where the waves break, and where
a more slender form could not hold its own. I have stood upon
a specimen of Xantha florida without crushing it.
Porcellana platycheles is a very small species, somewhat
dud and dirty in colour, and I have always found it on rocks
much grown over with algae, &c. Whilst P. longicornis, a
smooth, pink-coloured species, I have found upon smooth granite
boulders. Both these species have a wide distribution.
Pirimula denticulata is a small interesting species, and is
considered to be very rare. When collecting in Jersey, I