Hadley, Behavior of the American Lobster. 295 



orientation is produced, according to this tropism theory, by the 

 direct action of the stimulating agent on the motor organs of that 

 side of the body on which it impinges. A stimulus striking one 

 side of the body causes the motor organs of that side to contract 

 or extend or to move more or less strongly. This, of course, 

 turns the body till the stimulus affects both sides equally; then 

 there is no occasion for further turning and the animal is oriented " 

 (Jennings 1906a, p. 266). This is also brought out by Holt and 

 Lee (1901, p. 479), "The light operates, naturally, on the part 

 of the animal which it reaches." Thus, this tropism theory 

 requires that, in order to determine the direction of movement, the 

 stimulus must act more strongly on one side of the body than on 

 the other. It is needless to say also that in the majority of cases 

 the same conditions of stimulus which cause an animal to direct 

 the head away from the source of the stimulus, also determine 

 a movement in the same direction. Therefore, if we separate, as 

 has been done in this paper, hody-orientation from progressive 

 orientation, we can say that, in most organisms, the index of body- 

 orientation agrees with that of progressive orientation; the con- 

 ditions of stimulation which cause the one likewise determine 

 the other. Let us now see to what extent the behavior of the 

 larval lobsters agrees with these requirements of the local action 

 theory of the tropisms. In order to treat the matter concretely 

 we must consider it under two heads. First, body-orientation; 

 then, progressive orientation. 



It has been shown in the previous pages that, whatever the sign 

 of progressive orientation may be, the body-orientation is invari- 

 ably negative; and that this body-position is produced as a result 

 of diverse reactions which are attributable to the relative inten- 

 sities of light which strikes the eyes of the larvae. This body- 

 orientation, moreover, is consta t; it is not dependent upon the 

 age, stage, previous st mulation, hunger, "physiological state," 

 or upon any modifications of the external stimulus, such as changes 

 in intensity, duration of stimulation, etc. The orienting reaction 

 always comes about in the same way, so that we here have a case 

 where the "same-stimulus-same-reaction" principle invariably 

 holds. In other words, the reactions by which the larval lobsters 

 secure the characteristic body-orientation are typical and invari- 

 able motor-reflexes. 



Beyond producing the body-orientation, the direct motor-reflex 



