EARTHWORMS
Reading edition
In the preceding introduction it was stated that a certain degree of intelligence had been attributed to animals as low in the scale of evolution as Earthworms. No less an authority than Charles Darwin was responsible for this conclusion.[4] This great naturalist had observed that these lowly creatures had developed the peculiar habit of plugging up their burrows with dead leaves. He noted that leaves were drawn into the burrows by methods best suited to their particular shape. For example, linden leaves were drawn in by their tips, while no attempt was made to pull in leaves of the rhododendron by this method, these leaves being larger at the tip than at their base. On the other hand, pine needles, which frequently occur in pairs, with a common base, were never seized by the small end, since the separated ends of the needles would cause trouble. They were invariably attacked at the compact basal end, which is smaller than the mouth of the burrow.
“Instinct” may guide Worms in this nice discrimination. But Darwin tested them with materials unknown to them or to their ancestors. He cut up paper into triangular forms and placed it at the disposal of the Worms. To his astonishment the brainless creatures almost invariably seized the bits of paper by the most acute angle in carrying them to their burrows. Darwin therefore concluded that an intelligent choice of means to end had been made. Similar experiments were later made by Hanel, and Darwin’s results were verified; but the apparently intelligent reactions of the Worms were interpreted as “more or less complex reflexes in relation to the form and chemical nature of the objects drawn in.” As to which of these two interpretations is more in harmony with the observations, I leave to the reader’s own judgment.
Generally speaking, it is probably best to accept the guiding principle formulated by Prof. Lloyd Morgan, namely, “In no case may we interpret an action as the outcome of the exercise of a higher psychical faculty, if it can be interpreted as the outcome of one which stands lower in the psychological scale.”[5]
Darwin’s conclusion seems to be justified by some experiments quite recently carried out by Professor Heck of the University of Prague, who gathered together for his purpose some five hundred Worms. The Worms were introduced into a passage shaped like a capital T, carved from a block of wood, and covered with a glass plate so that the movements of the animals might be watched. When they came to the junction, about half of them turned one way and half the other. Then the apparatus was arranged so that those Worms which took the left-hand passage received a mild but probably disagreeable shock. At first they did not seem to know what to make of this; but after they had all been through the experience about 200 times, they nearly all took the right-hand turn. When the electrodes were then moved to the right-hand passage, the Worms learned to shift to the left-hand after only 65 trials—evidently showing something beyond the operation of mere chance.
Commenting on this attitude of Earthworms to learn by experience, a writer in The Scientific American (April, 1924) remarks:
“In the human sense, Earthworms have no brains; their nervous systems consist of a series of little ganglions, or nerve centers, on the under side of the Worms and connected with each other by nerve fibers. If the Worms were cut in two, the fragments still showed the ability to distinguish between the safe and the unpleasant road to travel, indicating that the Earthworm remembers in every one of its ganglions, and that it is able to learn and profit by experience.”
In other words, the phenomena of associative memory, hence the capacity to learn by experience, seems to occur not only in animals devoid of cerebral hemispheres—long regarded as the sole seat of intelligence—but in organisms possessing no cerebro-spinal nervous system. In Invertebrates the mechanisms which allow associative memory “will probably be found in the supra-œsophageal ganglion” (Loeb).
“The Earthworm has no specialized sense organs, it has neither eyes to see, nor nose to smell, nor ears to hear with. Still, although it is apparently deaf, it is not devoid of the power of appreciating those stimuli which in us excite the sensation of sight or smell. A strong light suddenly turned on the anterior end of the body will cause the Worm instantaneously to withdraw into its burrow, and Worms readily recognize the presence of such favorite food as onions and raw meat. Their sense of touch is well developed and they are very sensitive to vibrations; for instance, a stamp of the foot on the ground will cause all those in a certain radius to disappear into their burrows. It is further possible that Earthworms possess other senses with which we are totally unacquainted.”[6]
It is quite evident, from what has been said above, that remarkable responses to environmental stimuli are made by animals quite devoid of a brain and lacking in those sense organs popularly assumed to be necessary to such responses of the organism as have just been described.