The Hermit-crab deliberately seeks a partner-Anemone if one has not
chanced to attach itself to its adopted shell-home. Grasping an
Anemone with its claw, it places it upon the back of the Mollusk
(e.g., Periwinkle, Whelk) shell which it had appropriated for its
residence, and then adds a second and a third Anemone until it is
completely “camouflaged.” If the Crustacean has occasion to remove to
a new shell (which it does after each moult), it sometimes carries
a partner-Anemone on its great claw, as a form of protection. The
Anemone is, indeed, a sort of outer defense, being richly endowed with
batteries of stinging cells. Thomson remarks that the Hermit-crab’s
behavior is “suffused with an appreciative awareness of what he is
doing.”
Recent experiments conducted by Mr. S. Mikhailoff, of the Oceanographic
Institute at Monaco, on the Hermit-crab proved conclusively that a
complex neuro-psychical activity in animals having neither a cerebrum
nor a central nervous system is possible. The animal was able to
distinguish differences in shades of red, “refusing to respond to
anything except the color which was the ‘educating’ stimulus, even
when shades of red very near this were employed.” In Man the power to
distinguish between colors and their shades is located in the gray
matter of the cerebrum. The Hermit-crab has no cerebrum.
An important conclusion derived by Mr. Mikhailoff from his extensive
experiments is that it is a mistake to compare the ganglionic nervous
system of invertebrate animals to the sympathetic system of vertebrate
animals. He also proved by the experiments in question that it is
possible to establish an associated reflex “in response to any external
stimulus whatever.”[11]
It is interesting to note in this connection that Crabs, like Birds
and other animals, have their own peculiar homing instincts. Carry a
shore-crab back from the beach, lay it down, and unerringly it starts
back in the right direction, straight for the sea.
“Some land-crabs of the West Indies and North America combine in large
swarms to travel to the sea and to deposit therein their spawn; and
each such migration implies concert, co-operation and mutual support,”
remarks P. A. Kropotkin (“Mutual Aid,” 1902).