26 E. H. HARPER. 



strong enough to have the directive effect its extension movements 

 are random, an advance toward the light being checked and 

 orientation being brought about by following up of favorable 

 random movements. There are only two responses in reality, the 

 checking or drawing back of the head involving the symmetrical 

 use of the longitudinal muscles of both sides, and the turning 

 response, involving the longitudinal muscles of only one side, 

 that opposite to the source of stimulation. The two responses 

 may also be combined. 



THE ANATOMICAL BASIS FOR THE DIFFERENCE IN SENSITIVE- 

 NESS TO LIGHT IN THE EXTENDED AND IN THE 

 CONTRACTED STATE. 



The text figures introduced are intended to make clear the 

 reason for the difference in the sensibility to light of the anterior 

 end in the contracted and extended state. Hesse, who has 

 worked on the organs of light perception of the lower animals, 

 has shown the structure of the light cells in many species of 

 earthworms and has worked out their distribution segmentally. 

 He shows that these cells are most numerous on the first seg- 

 ment, and especially on the prostomium (which is fused with the 

 first segment in Perich&td) and that their number diminishes 

 rapidly on each segment as we go farther back. It is conse- 

 quently the very tip of the animal ( the posterior tip as well ) 

 which is most important for the perception of light, although 

 light cells are found in small numbers over the whole length of 

 the body. 



The sections of PcricJiceta (Figs. I and 2) show that the first 

 segments are subject to great extension and contraction. It was 

 not possible to get the worm fixed in the fullest state of either 

 extension or contraction. In Fig. 2 it is seen that the first seg- 

 ment is partly inrolled into the buccal cavity in the state of con- 

 traction. For further demonstration of this point the epithelial 

 layer alone, of the first segment, is represented in the extended 

 and the contracted state in Figs. 3 and 4. It is seen to be greatly 

 thickened as well as inrolled when contracted. The effect of 

 this on the light cells is seen by comparison of Figs. 5 and 6. 

 The light cells are on the basement membrane. The thickening 



