MOTOR RESPONSES IN INVERTEBRATES 589 



of increase in luminous intensity. There are therefore two types of 

 response, a rapid response which is closely correlated with the rate of 

 change in luminous intensity, and a slow response which is not closely 

 correlated with the rate of change in this intensity. Neither type of 

 response has been thoroughly investigated. 



In the epidermis and in enlargements, near the epidermis, of some 

 of the nerve cords, there are large cells, each of which contains a highly 

 refractive body surrounded by a nerve net. These cells are photo- 

 receptors. They are very abundant in the prostomium. From here 

 posteriorly they decrease rapidly to a minimum in about the sixth 

 segment, then remain nearly constant until near the posterior end where 

 they increase again. Their distribution is the same as the distribution of 

 sensitivity to light (Hesse, 107; Hess, 106). 



Lumhricus and Eisenia orient fairly precisely. They are usually 

 negative in all but very weak light in which they are positive (Adams, 1). 

 If they are laterally illuminated without increase of intensity, usually they 

 gradually turn directly from the light until they are oriented. If they 

 are laterally illuminated with increase of intensity, they frequently turn 

 directly toward the light, then swing the anterior end from side to side 

 for a time, and finally turn from the light. If the intensity of lateral 

 illumination is low, there is usually marked swinging of the anterior 

 end and orientation is indefinite. The swinging of the anterior end is 

 in the nature of exploratory movements ; it serves to localize the direction 

 of illumination more accurately. Turning from the light (orientation) 

 may be due entirely to contraction of the shaded side or entirely to 

 random swinging of the anterior end or to a combination of these two 

 phenomena. Contraction of the shaded side is directly correlated with 

 difference in the intensity of the illumination of opposite sides, but it is 

 probably in the nature of a reflex shock-reaction. At any rate, there 

 is no evidence indicating that it is due to difference in tonus of the 

 muscles on opposite sides in direct proportion to differences in the amount 

 of light received by the photoreceptors on opposite sides (Mast, 140). 



Specimens with the brain removed are positive in ordinary daylight, 

 i.e., they are positive in light of very much higher intensity than are 

 normal specimens. In specimens with the nerve cord cut, the portion 

 back of the cut is positive and that in front of the cut is negative. The 

 action of the brain, therefore, increases the negativity of the worm 

 (Hess, 105). Adaptation to light (Hess, 105), reduction in temperature, 

 injection of depressant drugs around the brain, and injury of certain 

 parts of the brain (Prosser, 187) have the same effect as removing the 

 brain; rise in temperature, within certain limits, and stimulating drugs 

 have the opposite effect. Specimens with the brain removed are less 

 active and become more readily light-adapted than normal specimens 

 (Prosser, 187). These facts indicate that decrease in metabolic processes 



