HERMIT-CRABS
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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).