HABIT FORMATION IN THE CRAWFISH CAMBARUS AFFINIS.[1]
BY ROBERT M. YERKES AND GURRY E. HUGGINS.
This paper is an account of some experiments made for the purpose of testing the ability of the crawfish to profit by experience. It is well known that most vertebrates are able to learn, but of the invertebrates there are several classes which have not as yet been tested.
The only experimental study of habit formation in a crustacean which we have found is that of Albrecht Bethe[2] on the crab, Carcinus maenas. In his excellent paper on the structure of the nervous system of Carcinus Bethe calls attention to a few experiments which he made to determine, as he puts it, whether the crab possesses psychic processes. The following are the observations made by him. Experiment I. A crab was placed in a basin which contained in its darkest corner an Eledone (a Cephalopod). The crab at once moved into the dark region because of its instinct to hide, and was seized by the Eledone and drawn under its mantle. The experimenter then quickly freed the crab from its enemy and returned it to the other end of the basin. But again the crab returned to the dark and was seized. This was repeated with one animal five times and with another six times without the least evidence that the crabs profited by their experiences with the Eledone. Experiment 2. Crabs in an aquarium were baited with meat. The experimenter held his hand above the food and each time the hungry crab seized it he caught the animal and maltreated it, thus trying to teach the crabs that meat meant danger. But as in the previous experiment several repetitions of the experience failed to teach the crabs that the hand should be avoided. From these observations Bethe concludes that Carcinus has no 'psychic qualities' (i.e., is unable to profit by experience), but is a reflex machine.
Bethe's first test is unsatisfactory because the crabs have a strong tendency to hide from the experimenter in the darkest corner. Hence, if an association was formed, there would necessarily be a conflict of impulses, and the region in which the animal would remain would depend upon the relative strengths of its fear of the experimenter and of the Eledone. This objection is not so weighty, however, as is that which must obviously be made to the number of observations upon which the conclusions are based. Five or even twenty-five repetitions of such an experiment would be an inadequate basis for the statements made by Bethe. At least a hundred trials should have been made. The same objection holds in case of the second experiment. In all probability Bethe's statements were made in the light of long and close observation of the life habits of Carcinus; we do not wish, therefore, to deny the value of his observations, but before accepting his conclusions it is our purpose to make a more thorough test of the ability of crustaceans to learn.
Fig. 1. Ground Plan of Labyrinth. T, triangular compartment from which animal was started; P, partition at exit; G, glass plate closing one exit passage. Scale 1/6.
For determining whether the crawfish is able to learn a simple form of the labyrinth method was employed. A wooden box (Fig. 1) 35 cm. long, 24 cm. wide and 15 cm. deep, with one end open, and at the other end a triangular compartment which communicated with the main portion of the box by an opening 5 cm. wide, served as an experiment box. At the open end of this box a partition (P) 6 cm. long divided the opening into two passages of equal width. Either of these passages could be closed with a glass plate (G), and the subject thus forced to escape from the box by the choice of a certain passage. This box, during the experiments, was placed in the aquarium in which the animals lived. In order to facilitate the movement of the crawfish toward the water, the open end was placed on a level with the water in the aquarium, and the other end was raised so that the box made an angle of 6° with the horizontal.
Experiments were made under uniform conditions, as follows. A subject was taken from the aquarium and placed in a dry jar for about five minutes, in order to increase the desire to return to the water; it was then put into the triangular space of the experiment box and allowed to find its way to the aquarium. Only one choice of direction was necessary in this, namely, at the opening where one of the passages was closed. That the animal should not be disturbed during the experiment the observer stood motionless immediately behind the box.
Before the glass plate was introduced a preliminary series of tests was made to see whether the animals had any tendency to go to one side on account of inequality of illumination, of the action of gravity, or any other stimulus which might not be apparent to the experimenter. Three subjects were used, with the results tabulated.