132 re;ports on investigations and projects. 



(i) Most of the turbellaria have numerous eyes, or eye-spots, two of 

 which are situated near the middle of the anterior end and the rest near the 

 edge of this end extending some distance back. In one of the species studied 

 only two eyes were found, and these were cut out in a number of specimens 

 which had been quieted in an isotonic solution of magnesium sulphate. 



Nearly all of these animals recovered and the wounds were healed in two 

 days. They responded to light by becoming active when exposed in strong 

 illumination, but no indication of orientation in any of them was observable 

 until after the eyes had regenerated ; and the same results were obtained with 

 specimens of the other three species in which the eyes were removed by cut- 

 ting off the entire anterior end. This end, in spite of its diminutive size, 

 oriented very soon after the operation, but the remainder, the portion which 

 contained no eyes, showed no indication of orientation in any instance, al- 

 though to an increase of intensity it responded definitely by becoming more 

 active and sometimes raising the anterior end and swinging it from side to 

 side. These results show clearly that the eyes of the marine turbellaria func- 

 tion in the process of orientation. 



(2) As to the nature of the stimulus, we wish to consider only the ques- 

 tion : Are the turbellaria stimulated by light owing to an effect produced by 

 virtue of a change of intensity or to some effect produced by virtue of con- 

 stant or continued intensity, i. e., the absolute amount of energy received at 

 a given time as compared with the amount previously received ? 



All of the species of marine turbellaria tested, as already stated, become 

 more active when the light intensity is increased. This is true for specimens 

 with the eyes removed as well as for others. But the increase in activity 

 does not take place until some time after the intensity of the light is changed. 

 This seems to indicate that the stimulus which regulates the rate of locomo- 

 tion is dependent upon the amount of energy received, that is, constant in- 

 tensity, not change of intensity. 



With reference to the nature of the stimulus which regulates orientation 

 the evidence is not conclusive, although it indicates that it is due to change of 

 intensity. 



Planaria with one eye removed, either by gouging it out or by cutting off 

 one side of the anterior end obliquely, turn continuously from the wounded 

 side for some time, evidently owing to the stimulation of the wound, since 

 after this is healed they tend to turn in the opposite direction. After regen- 

 eration is nearly complete they orient practically as accurately as normal 

 specimens. But if the light intensity is suddenly increased after the animals 

 are oriented in a given beam of light, without any change in the direction of 

 the rays, they turn sharply from the side containing the regenerated tissue. 

 The fact that the planarians turn in this direction indicates that the regener- 

 ated tissue is more sensitive to light than the old tissue ; and that it is probable 

 that the orienting stimulus in these animals is due to change of intensity — 

 that there is no stimulation resulting in orientation unless the intensity on 

 some part of the body changes. This is usually produced by the shading of 

 one part of the body by another (probably the sensitive part of the eye by the 

 opaque part) owing to the change in the axial position of the organism. 



Planaria exposed at the intersection of two beams of light which cross at 

 right angles orient with the anterior end directed from a point between the 

 sources of the two beams. If, after the animals are oriented, the light in one 

 of the beams is intercepted so as to decrease the intensity on one side of the 

 organisms, they usually turn directly toward the side on which the intensity 

 was decreased until they are oriented in the single beam of light. If the 

 light in the second beam is now thrown upon the animals so as to increase 



