e the weed itself. "The shrimps and crabs which swarm in the weed are
of exactly the same shade of yellow as the weed, and have white markings
upon their bodies to represent the patches of Membranipora. The small
fish, Antennarius, is in the same way weed-colour with white spots. Even
a Planarian worm, which lives in the weed, is similarly yellow-coloured,
and also a mollusc, Scyllaea pelagica." The same writer tells us that "a
number of little crabs found clinging to the floats of the blue-shelled
mollusc, Ianthina, were all coloured of a corresponding blue for
concealment."[76]

Professor E.S. Morse of Salem, Mass., found that most of the New
England marine mollusca were protectively coloured; instancing among
others a little red chiton on rocks clothed with red calcareous algae,
and Crepidula plana, living within the apertures of the shells of larger
species of Gasteropods and of a pure white colour corresponding to its
habitat, while allied species living on seaweed or on the outside of
dark shells were dark brown.[77] A still more interesting case has been
recorded by Mr. George Brady. He says: "Amongst the Nullipore which
matted together the laminaria roots in the Firth of Clyde were living
numerous small starfishes (Ophiocoma bellis) which, except when their
writhing movements betrayed them, were quite undistinguishable from the
calcareous branches of the alga; their rigid angularly twisted rays had
all the appearance of the coralline, and exactly assimilated to its dark
purple colour, so that though I held in my hand a root in which were
half a dozen of the starfishes, I was really unable to detect them until
revealed by their movements."[78]

These few examples are sufficient to show that the principle of
protective coloration extends to the ocean as well as over the earth;
and if we consider how completely ignorant we are of the habits and
surroundings of most marine animals, it may well happen that many of the
colours of tropical fishes, which seem to us so strange and so
conspicuous, are really protective, owing to the number of equally
strange and brilliant forms of corals, sea-anemones, sponges, and
seaweeds among which they live.

_Protection by Terrifying Enemies._

A considerable number of quite defenceless insects obtain protection
from some of their enemies by having acquired a resemblance to dangerous
animals, or by some threatening or unusual appearance. This is obtained
either by a modificati