REPORT OF COMMISSIONERS OF INLAND FISHERIES. 201 



somewhat at variance with the majority of observations on the 

 phototactic reactions of most animals, as it is contrary to the condi- 

 tions of body orientation which we find in the fourth larval stage of 

 the lobster itself ; for in this stage we find, at least in the phototactic 

 reactions, that the body orientation brings the head toward the 

 source of illumination rather than away from it, as is invariably the 

 case in the first three larval stages. 



A question at once arises as to what we may mean by a positive 

 photatactic reaction, for in the case of the lobster the fact is clear 

 that we may very frequently have a negative body orientation coupled 

 with a positive progressive orientation. Loeb* states: "Positively 

 heliotropic animals are compelled to turn their oral pole toward the 

 source of light and to move in the direction of the rays to its source." 

 The instance of the lobster larvae, and of other forms mentioned 

 above, are certainly notable exceptions to this general rule. Until 

 we know more regarding these differences and regarding the rela- 

 tions between body orientation and progressive orientation, it may 

 be considered safe to say that the direction of the progressive movement, 

 with respect to the source of illumination, may be held as the surest 

 criterion for the nature of the phototactic response of animals in 

 general. 



On the other hand, the point has been made clear by some writers,, 

 that the body orientation of the organisms, the definite relation of 

 the longitudinal body axis to the lines of active force of the stimulus,. 

 is the primary consideration for all deeper problems dealing with 

 progressive orientation. However 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 orienta- 

 tion per se has little or nothing to do icith the question of the positive 

 or negative progressive orientation of the organism; for the same condi- 

 tions which invariably determine a negative body orientation may also 

 determine either a positive or negative progressive orientation, as other 

 circumstances demand. We will therefore first concern ourselves 



♦Studies in General Physiology. Decennial Reports of the Univ. of Chicago. Vol. XX. p. 30. 



