ON THE BEHAVIOR AND REACTIONS OF LIMULUS IN EARLY STAGES OF ITS DEVELOPMENT.

BY RAYMOND PEARL.

[Reprinted from THE JOURNAL OF COMPARATIVE NEUROLOGY AND PSYCHOLOGY, Vol. XIV, No. 2, April, 1904.]

ON THE BEHAVIOR AND REACTIONS OF LIMULUS IN EARLY STAGES OF ITS DEVELOPMENT.1

By RAYMOND PEARL.

Introduction.

In morphological research two modes of procedure are usually followed in investigating the significance of some particular structure of an organism. First the form, position, relations and other characteristics of the structure in the adult organism are studied. Then the embryological history is worked out for the purpose of ascertaining how the structure develops to the complex condition of the adult. In this way,

of course, has been gained the complete explanation of many organs and structures which were inexplicable when the adult. condition alone was considered. Indeed, embryological study has come to be considered an absolutely necessary part of almost any morphological investigation which aims at completeness. The ontogenetic history of an organ is regarded as of prime importance in elucidating the adult condition.

It is evident that the same thing may be true when the problem under consideration is one in animal behavior, instead of in animal morphology. As we go up the scale from the lower to higher forms, the behavior becomes more and more complex, and less easily resolvable into simple component factors. To be sure, the increase in physiological complexity does not run exactly parallel to the increase in morphological complexity, yet one does not have to go far before the analysis of the behavior of the adult organism becomes extremely diffi

1 Contributions from the Zoological Laboratory of the University of Michigan, Ann Arbor, Mich., No. 71.

cult of accomplishment. For some time before the present piece of work was begun it was the opinion of the writer that valuable aid in the analysis of the behavior of higher organisms might be gained by following the plan of the morphologist and studying the developinent in the individual of the characteristic features of the behavior. Just as the morphologist studies the ontogeny of an organ as an aid to the understanding of the adult condition, so might the comparative psychologist study the ontogeny of a reaction. It seemed reasonable to suppose, in view of the close relationship which JENNINGS and others have shown to exist between structure and type of behavior in lower forms, that in higher forms the behavior would be simpler in character during embryonic or larval life when the structure is simpler. Of course we know in a general way that this is true; but does it hold in detail for single complex reactions and reflexes? So far as is known to the writer, very little systematic work on behavior has been done from this point of view, except on some of the mammals and birds (cf. notably the work of MILLS, LLOYD MORGAN and SMALL). In these forms the behavior has evidently a considerable psychical element in it. It was with the idea of determining whether anything of importance might be gained by studying the ontogeny of reactions primarily reflex in nature that the present piece of work was undertaken.

The form chosen for study was the king-crab Limulus polyphemus. The reasons for this choice were two-fold; in the first place, I was already familiar with the behavior and reactions of the adult organism, and in the second place, Limulus is a form in which the behavior is quite complex, and yet at the same time the different reflexes are strikingly definite and machine-like in character. The adult Limulus is an almost ideal form for physiological work, on account of its tenacity to life after most extensive operations have been performed upon it, and because of the definiteness of its responses. Something of the complexity as well as the definiteness of its behavior can be gathered from the excellent account which PATTEN ('93) has given of the gustatory reflexes, for example.

The behavior of the adult Limulus is principally made up of the following movements and reflexes: the respiratory movements of the abdominal appendages, the swimming movements of the abdominal and thoracic appendages, various "gill cleaning" reflexes of the abdominal appendages and the sixth legs, walking movements, and the reflexes of gustation and deglutition. Furthermore, it responds to temperature stimuli very strongly and characteristically, also to certain sorts of tactile stimuli, to disturbances of its equilibrium, to chemical stimuli, and is thigmotactic and phototactic. The original plan of the present work was to make a thorough detailed study of the behavior and reactions during every stage of development from the time the embryo left the egg membranes till it attained the adult condition, in its behavior at least. It was hoped that in this way steps in the development of the reactions and reflexes could be traced.

The work was begun in the U. S. Fish Commission Laboratory at Wood's Hole, in July, 1900, and continued throughout the summer. In that time the behavior was studied and the work finished from my point of view (with the exception of the phototactic reaction) up to the stage at which the first moult occurs (end of the so-called "trilobite" stage). During the following fall a preliminary statement of the results obtained was published in Science.' It was expected at that time that the work would be taken up again the next summer and older stages studied. This, however, proved to be impossible and at no time since have I been able to take up the work again. As it is uncertain when I shall be able to go on with this problem it has seemed desirable to publish the results so far obtained. I wish to put on record the complete statement of the facts made out in the two developmental stages which have so far been investigated.

It gives me pleasure to make acknowledgement at this point to those who have in one way or another aided in this work. To Professor WM. PATTEN, my friend and former

1 N. S. Vol. XII, No. 311, pp. 927-928, 1900.

teacher, I am indebted, not only for the material on which the work was done, which he very kindly furnished me, but also for many helpful suggestions freely offered as the work progressed. When the work on Limulus embryos was begun I had already been engaged for two years on a study of the physiology of the brain of the adult animal, under his direction. Without the thorough knowledge of the adult behavior thus gained the present study would hardly have been possible. To the authorities of the U. S. Fish Commission, and especially to Dr H. C. BUMPUS, I am indebted for the numerous facilities which were freely placed at my disposal at the Wood's Hole Laboratory,

Material and Methods.

The material used consisted of several hundred developing Limulus eggs which were given me by Dr. PATTEN. When received they were nearly all at about the stage of development, designated as Stage I, by KINGSLEY ('92). At room temperature development proceeds quite rapidly, but owing to the fact that there is a great deal of variation in the rate of development, individuals in widely different stages of development may be found at any time in the same batch of eggs. The eggs and embryos were kept in shallow glass dishes in sea-water which was changed at intervals, usually once in twenty-four hours. In this way

the embryos were kept in good condition for a period of nearly two months.

The results of the present work will be discussed under two headings first the behavior and reactions before the em. bryo leaves the egg membrane ("vicarious chorion," KINGSLEY'S Stage I), and second, the behavior up to the time of the first moult after the animal begins its free existence (the so-called "trilobite" stage, KINGSI EY's Stage K). General accounts with figures of the morphological development of Limulus are given by KINGSLEY ('85 and '92).

The Behavior before the Embryo begins Its Free Existence.

Stage of Development.—The earliest stage at which definite results could be obtained regarding the movements and reac