Hadley, Behavior of the American Lobster. 261 



reste le meme, le sens du deplacement peut changer." Lyon 

 (1906) has recorded a similar observation for several larval stages 

 of Palemon. This condition of affairs is rather at variance with 

 the majority of observations on the phototactic reactions of animals 

 and it is contrary to the condition of body-orientation v^hich we 

 find in the fourth stage of the lobster itself, for in this stage (at 

 least in some of the assumed phototactic reactions) the body-orien- 

 tation brings the head toward the source of illumination instead 

 of away from it as is invariably the case in the first three stages. 



The question has already arisen as to what we may mean by a 

 positive phototactic reaction, for in this case it is clear that we may 

 very frequently have a negative body-orientation coupled with a 

 positive progressive orientation. Until we know more regarding 

 the differences between body-orientation and progressive orienta- 

 tion, It may be considered safe to say that the direction of the pro- 

 gressive movement, with respect to the source of illumination, may 

 be held as the surest criterion of the sign of the phototactic response 

 of animals. On the other hand the point has been made clear by 

 some writers, that in the body-orientation of organisms the definite 

 relation of the body-axis to the lines of active force is the primary 

 consideration for all problems of progressive orientation. How- 

 ever this may be, we have before us at least one instance wherein, 

 although the relation of the body-axis to the lines of force is an 

 important consideration, the body-orientation per se has little or 

 nothing to do with the question of the positive or negative progress- 

 ive orientation of the organism; for as we have already observed, 

 conditions which invariably determine a negative body-orientation 

 may determine either a positive or a negative progressive orienta- 

 tion, as other circumstances demand. We may, therefore, first 

 concern ourselves with the mechanics of progressive orientation 

 and then turn wath better understanding to the mechanics of bod y- 

 orientation, for these two reactions apparently depend upon quite 

 different circumstances. 



2. The mechanics of progressive orientation — The only means 

 of locomotion possessed by the larVcX of the first three stages are the 

 exopodites of the thoracic appendages and the strong, flexible 

 abdomen with its broad terminal fan (Fig. i). It is but seldom, 

 however, that the latter is used, and never when it is a question 

 of progressive orientation to light. We are then confronted with 

 the problem: How, by the motion of the thoracic exopodites 



