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by two oesophageal branches with the central nerve cord, which is represented principally by a single large thoracic ganglion or concrescence of ganglia. The thoracic portion of the nervous system is, however, symmetrical. The mandibles, or the third pair of appendages, crushing jaws, the right of which is larger, the maxillae and the maxillipeds all receive nerves. ous system is therefore in general so constructed that it would seem at least reasonable to expect that associations might be formed

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Experimental The systematic experiments by which the association between the “constructs” of two sense fields, taste and vision, was established, and a "reconstruction" or reproduction subsequently shown possibly to take place, were preceded by various preparatory observations, some of which were made in the summer of 1902. For instance, it was then shown that the Hermit is remarkably thigmotactic, for when a shell inhabited by a crab is suspended at the distance of about twice the diameter of the shell from the floor of the aquarium, the animal is thereby made decidedly uncomfortable, protrudes nearly its entire body, feels about, and usually leaves its shell, especially if there is a vacant shell near by. Suspended at the height of from eight to ten inches the crab will remain in the shell until it dies. They are also somewhat rheotactic. New shells thrown into the aquarium are soon examined and accepted at what would sometimes seem to be a disadvantage. This constant "desire" for change, to

, gether with both a great natural rapacity and pugnacity, are indeed indications of a strenuous life even among Hermits.

Both that series of observations upon which special emphasis is placed in this paper, and that preliminary one which showed that the method adopted would probably lead to satisfactory results, were made with a very simple apparatus and in a very simple way. A number of crabs which had been kept in an ordinary laboratory glass jar aquarium about twenty inches in diameter, were made to go into a darkened portion of this that they might get their food, which consisted of a freshly

cleaned Fundulus held in place on a wire. The same portion of the aquarium was darkened each time just before feeding by setting down into the constantly running water a screen consisting of two thin boards fastened at right angles, leaving only an opening at each end of the vertical board wide enough for the crabs to go through one by one; around the outside of the aquarium from end to end of this portion set off by the screen was kept a piece of heavy brown paper to shut out the light coming from the other direction. The only light which could enter came therefore through the openings at each end. Sand was placed in the bottom of the aquarium, and all the conditions such as its position and that of the tap, with the exception of the putting in and taking out of the wooden screen, were kept constant during the entire series of experiments.

These were conducted in detail as follows: to establish, first, an association and, second, present an occasion for possible "'reproduction" from the “after-effect" of one by the external stimulus of the other construct.”

Thirty-six crabs of the species Eupagurus longicarpus were placed in the aquarium on July 30th, and, first, allowed until August 6th to become accustomed to aquarium conditions ; during this period they were simply fed each day with a fresh Fundulus, no screen being used; they seized their food most eagerly, oftentimes fighting and driving each other away from it. The death of six selected the thirty most fit individuals. The crabs were, furthermore, observed to remain in the lightest part of the aquarium practically all of the time, i. e., they were positively heliotropic. The positive heliotropism was confirmed by a number of control experiments with other lots and by that of Aug. 6th. The screen was inserted and all the crabs, 30 in number, placed behind it. In 10 minutes 28 had gone through the opening into the light and 27 of these were near the point of its maximum intensity.

Each day following this, the screen was inserted, a fish on a wire placed in the darkened portion, and the number of crabs. going into the dark through either one of the entrances at the end within a given time, which was constantly shortened during the series, counted. A crab going in and coming out again was counted as in, but in every case by far the greater number that had entered remained behind the screen as long as the food was there. Two stimuli to two different senses, taste and vision were thus simultaneously and contiguously presented ; and if this led to an association between the two constructs,' "food" and "screen-darkness," it would necessitate the overcoming of the natural positive heliotropism manifested at the start, and by contrast on this background the association would stand out more prominently. After the crabs were fed each

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1 This long time was granted in order, if possible, that the crabs might ultimately find their way in as a result, perhaps, of merely wandering around in their evident endeavors to localize the source of the taste stimulus, which in every case caused considerable agitation. The varying length of time used throughout the series was a matter of best adapting the means to the end. Thus it was quite justifiable to shorten the time later to 3' if the majority of the crabs went through the openings in that time, although to do this would give manifestly more favorable results than to wait longer.

? On this 5th. as well as on each successive day up to the gth. it was noticed that as soon as the screen was put in, and the fish was placed behind it, the crabs were much agitated and some started for the openings.

3 The three that did not go in on the previous day had been removed for the purpose of determining it possible if there was already a certain permanency of habit among the twenty-seven entering; the result was confirmatory. The three were then returned.

* One had died.

day under these conditions, fish and screen were removed, and the latter was carefully washed with running sea water, as it was also each time just before using. After a few days of this treatment immediately upon the insertion of the screen the

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crabs became most agitated, some hurrying and scurrying about, others making almost directly for one of the openings. A preference for the right-hand one seemed to exist and this may nected with the right-handed asymmetry. No attempt however to investigate this matter systematically was made, and there were no “landmarks" in the aquarium, such as stones, etc., whereby a path could be learned. The preceding tabulated results show, (1) the total number each day, (2) the number going into the dark within (3) a certain time, when fed under the condition named, (4) the per cent. entering, and (5) for comparative purposes, the per cent. entering in one minute.

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| At the end of the 5' the fish was put behind the screen in order that the association might not be broken up by the crabs entering and finding nothing there. When this was done all but one of the eight outside, four of which had gone in and come out again, entered within 2'. This was done each successive day.

? Then fed, and in i' all but one had entered. This shows again the constant improvement, the learning taking place in a surprisingly short period.

3 In one minute after feeding all of the seven outside had entered.
* After feeding all entered in 1/2'.
5 After feeding all but one entered in 2'.

Accordingly, after the 8th. day, evidence having been thus secured that an association between the two “constructs," food and sense-darkness, had been established, its efficiency was

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further tested by simply putting in the screen with no fish be. hind it, and the record of the crabs entering was taken in the same way as before. The results are tabulated above.

These results, if confirmed by control experiments, must, we think, be accepted as showing conclusively that the Hermit crab of the species longicarpus, firstly, forms an association between two sense constructs, which, secondly, can be interpreted as showing that the crab, subsequently, when only one stimulus is presented, reproduces an image of the other. The same reaction, entering the dark, which previously demanded two stimuli, is later secured with only one stimulus; the other therefore must either be excited or reproduced. We may say perhaps that if, when only the screen is put in as in the second series, only visual perception or recognition takes place, then there is no reason why the crabs should not remain where they are, in the light, which is their natural preference. The screen, which they now recognize, has however through association come to mean for them other than something to be avoided; it means “food,” and this meaning is present when the food is not. The difference between YERKES' experiments and these consists, therefore, in this, that YERKES' crabs “acquired the habit” of going by a correct path from a place disliked to a place liked ; the Hermits on the other hand go from a place liked to a place naturally disliked, but artificially” liked because of food either there or-may we say, "expected” to be there. This must mean that an associative element at first external, i, e., physical, but now no longer that, is, nevertheless, now present as internal, and its internal presence must be due to either an excitation or a reproduction by the other stimulus.

If the latter, then the Hermit may be said to remember vaguely, i. e., to reconstruct.

These conclusions are strengthened by the following control experiments :

EXPERIMENT III. Lot 4. Aug. 20th. Forty crabs in a similar aquarium ; the same screen was used, carefully washed each time. The previous procedure was reversed here, by placing the crabs be

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