172 The Animal Mind 



mal's physiological condition, is to be found as early as the 

 work of Pouchet on fly larvae. He noted that the courses 

 taken by the larvae away from the light were either straight, 

 "or they present to right and left indentations due to the 

 wavering movements which the animal makes ... in a 

 certain number of cases, as if to take at each instant a new 

 direction." These individual differences might have been 

 accounted for, says Pouchet, by differing degrees of hunger 

 in the larvae (347). 



Phototaxis in certain tube-dwelling marine worms was ob- 

 served by Loeb. Spirographis spallanzanii gradually curves 

 its tube until its oral end faces the direction from which the 

 rays of light come ; and another marine worm, whose tube is 

 absolutely stiff, adapts itself to a change in the direction of 

 the rays by curving the newly -formed portions of the tube as 

 it constructs them (236). 



Attempts to show the independence of photopathy and pho- 

 totaxis by causing a positively phototactic animal to move 

 toward the source of light even when, by an arrangement of 

 screens overhead, such movement brings it into a region of 

 dimmer illumination, have been made with apparent success 

 on the crustacean Daphnia (93). That no increase in the 

 intensity of the light will reverse Daphnia's positive phototaxis 

 is also evidence that photopathy, the seeking of an optimum 

 intensity, is absent in these Crustacea (457). Simocephalus, 

 being made to collect in the brighter regions of a trough and 

 showing no orientation to light rays entering the trough at 

 right angles, seemed to display photopathy independent of 

 phototaxis (448). It is very difficult, however, to be sure in 

 such experiments that the direction of the light rays and the 

 intensity of the illumination are really independently varied, 

 for the diffusion of light by floating particles and its reflection 

 from the sides of the trough offer disturbing factors. The 



