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AN ESTABLISHMENT OF ASSOCIATION IN HERMIT
CRABS, EUPAGURUS LONGICARPUS.
By E. G. SPAULDING, Ph. D. (Bonn).
College of the City of New York.
The experiments described in this paper are the result, first, of preliminary observations of a number of hermit crabs kept for some time in an aquarium at the Woods Hole Laboratory, which showed them to be quite capable of profiting by experience. In fact, the results first obtained were in general quite confirmatory of those obtained by the subsequent more systematic investigation, the method for which they indicated.
Bethe' and YERKES' have each made experimental studies of habit formation in the Crustacea, the former on the crab, Carcinus moenas, the latter on the crawfish, Cambarus affinis, and on the green crab, Carcinus granulatus. BETHE at the end of his paper relates some experiments made to determine whether or not the crab possesses psychical processes, with the result that he asserts that it does not. This conclusion is not, however, necessarily to be accepted even from Bethe's own experiments, for the reason that these at best serve to demonstrate the absence of only one kind of psychical phenomena, viz., those of inhibition or control; other kinds may be present. BETHE himself does not recognize that the method he employed was defective in this respect, but an account of it will, we think,
BETHE, A., Das Centralnervensystem von Carcinus moenas. Archiv f. mikr. Anat, Bd. 51, 1898.
• Yerkes, ROBERT M. and Huggins, Gurry E. Habit Formation in the Crawfish, Cambarus affinis. Harvard Psychological Studies, Vol. I. 1903.
YERKES, Robert M. Habit Formation in the Green Crab, Carcinus gran. ulatus. Biolog. Bulletin, Vol. III. 1902.
make evident the justification of our criticism. His first experiment was to place a crab in a basin in the darkest corner of which there was an Eledone (a cephalopod). The crabs, because of their instinct to hide, moved immediately into this corner and were seized by the Eledone. Freed from its grasp one crab returned repeatedly five times, another six, to the dark and the enemy, showing, as BETHE thinks, that it had not profited by experience. It is to be emphasized, however, that to have done this latter in the way. BBINE thought possible, it would have been necessary that the crab whibit its instinctive action. This inhibition gauld take place only if, first, a representation of the pain of the seizurë bị the Elddone were present, and second (and essentially), if the, représentation were the “stronger”; the other possibility, that the representation should occur and yet be overcome by the instinct, is accordingly not disproved by BETHE's experiment. The same criticism applies also to his second method that, notwithstanding maltreatment on each such occasion, the crabs repeatedly seize food when offered.
The criticism above made is quite in agreement with that principle of method for comparative psychology which is in reality very simple, but not always observed, that in any instance where the question of the presence of consciousness in any species is admittedly to be decided by experimentation, this question must take a particular form, and our efforts must be directed to the establishment of the presence or absence of some definite kind of consciousness, e. g., associative memory between constructs of two sense fields, conceptual reasoning, etc.
YERKEs in his experiments with the crawfish made use of the labyrinth method. The subject could escape from a box into the aquarium only by "choice of a certain passage.' The “choice" consisted in or was manifested by learning (by repeated experience) to avoid the blocked passage and gain the aquarium by the most direct path. Accordingly all conflict, requiring inhibition, between the two elements or “constructs" to be associated, viz., "correct path" and "aquarium," was absent. “Correct path,” as opposed to the incorrect, logically implies in these experiments “aquarium”; and its selection, as shown by the ratio of improvement from day to day may, though not necessarily, imply the representation of the construct "aquarium," but it does demand the admission that acts of recognition and discrimination, or even of what Lioyd MORGAN calls “perceptional inference" take place. These in turn presuppose necessarily, as is well known, retention and production.
Carefully excluding the possibility of the crab's merely following a path by smell, taste, or touch (although if it did only this one could not account for a correct after an incorrect choice had once been made) YERKES found in one case that after 40, in another that after 250, experiences no mistakes in choosing were made. In a number of cases the subject turned from, before it reached, the partition which blocked the passage, thus showing the important part played by vision in directing the animal in the absence of smell, taste, and touch. A11 of these, however, together with muscular sensations, YERKES concludes normally play a part in the formation of labyrinth habits. These experiments therefore seem to show that upon the basis of the “constructs" which one sense alone, viz., vision, give the crab, a consistent selection of the correct path is possible ; but this is explainable it seems, even if it is considered that only a recognition of each successive part of that path and consequently a discrimination between it and the incorrect is made, and yet that no representation or “reconstruction" of “aquarium" takes place, although of course this latter interpretation is not excluded.
A method of experimentation, however, which shows that in the formation of a habit, or in the learning of a motor reaction involving two sense fields, e. g., taste and vision, it is necessary to overcome an instinct or tropism in the opposite direction, such a method, we think, would at least give more cogent grounds for accepting the presence of representation than one not doing this, although even here conservatism in making this claim would be the safer course.
Some Characteristics of the Hermit Crab. The genus Eupagurus is easily found in the shallower waters about Woods Hole and is represented by four species, longicarpus, annulipes, acadianus, pollicaris. E. longicarpus was selected for the present investigation on account of its convenient size (374-1/2 inches in length) for aquarium purposes, and because of a manifestly greater brightness. Supplementary experiments show that E. pollicaris, e. g., learns with greater difficulty.
Members of the entire genus inhabit, under normal conditions, the shells of gastropods, by which the abdomen is completely protected, the cephalothorax alone protruding. This peculiar mode of life is correlated with a dextral asymmetry, which extends to almost all the organs of the entire body, and which shows a very nice adaptation. This favors the view that the asymmetry is a result of life in dextrally spiral shells, exemplifying at the same time degeneration.
The establishment of the fact that these Hermits learn is not surprising in view of the complexity and fineness of their physiological sense apparatus, which is essentially the same as that of all the Crustacea, so that it is very probable that any denial of this ability to any species of the group, even upon the basis of experiment, is due to incomplete or faulty methods of investigation.
Sense Organs. The crab has only two general kinds of sense organs, viz., eyes and sense hairs, the latter of which are, however, differentiated as to their function. These hairs, which are found in all the extremities, are epithelial in nature, and are not penetrated by a nerve, but rather this latter spreads out underneath each epithelial group and gives to each cell a fibril. These epithelial sense cells lie in a support of “Matrixzellen,”' and according to variations in their structure and especially position are respectively gustatory, tactile, and auditory or “equilibra
I vom Rath, Orto. Zoologischer Anzeiger, No. 386, 1892.
tory." The gustatory hairs, lie as two patches of "minute curi. ously flat organs” on the under surface of the outer filament of the antennules, the innermost appendages, which observation shows are kept moving constantly. There are no sense organs in the mouth.?
At the basis of each antennule is a little sac formed by an infolding of the chitinous integument, communicating freely with the water, and containing little sand grains or otoliths. Here are present a second kind of sensory hairs, connected with the central nervous system by branches of the antennulary nerve, and whose function is either that of audition or of equilibrium.' The third class of hairs are tactile in function, and are especially numerous on the antennae, i. e., the second pair of appendages, although there are some on the antennules, the other appendages, and the remaining integument.
The two eyes of the crab are compound or facetted and are seated on movable pedestals. They are covered by a transparent chitinous cuticle, forming a cornea; this is divided into facets, beneath each of which there is an ommatidium with two segments (a) an outer, which is vitreous and refractive, and an inner, a short retinula, which is sensitive, thus giving a structure analogous to rods and cones. These cones are surrounded and so separated from each other by a pigment; their apex is embraced by elongated cells in the midst of which is a fibril of the optic nerve. Each facet functions as a single eye and therefore like the vertebrate eye gives no sensitive continuum but, rather, “mosaic vision," i. e., various images in juxtaposition. The eye as a whole is supposed to give a vision of distinct objects and space relations.
The brain is formed from the first three pairs of embryonic ganglia, and is therefore a "syn-cerebrum”; it supplies the eyes, the antennules and the antennae with nerves. It is connected
VOM RATH, Orro. Zur Kenntniss der Hautsinnesorgane der Crustaceen, Zoologischer Anzeiger, 365, 1891.
* HENSEN (Studien über das Gehörorgan, Zeitsch. f. wiss. Zoolog., Bd. 13, 1863) says they are auditory, while Delage, Archiv. d. Zool. Expér., 1887, (2), T. 5, says they are for position and qeuilibrium. (ited by vom Rath.)