464 C. M. Child. 



with Phagocata and Dendroccelum, Lillie ('01) records the fact 

 that while Phagocata equals Planaria in its regenerative power, 

 the conditions in Dendroccelum are widely different. In this 

 form the capacity for regeneration of a head is limited to the 

 anterior third or fourth of the body, pieces from levels posterior 

 to this failing to regenerate. Lillie noted that the pieces of Den- 

 droccelum which were incapable of regenerating a head showed 

 a marked difference in reactive power from those in which such 

 regeneration was possible. On the other hand it is known (Loeb, 

 '94,'99, Parker and Burnett, '00) that pieces of Planaria deprived 

 of the cephalic ganglia react to stimuli in much the same manner 

 as normal animals. In view of these differences in reactive power 

 Lillie suggests that the stimulation of the normal movements may 

 determine the fate of the undifferentiated mass of new tissue, the 

 head failing to regenerate in the absence of the characteristic 

 stimuli. This suggestion is, I think, an important one. 



As Lillie points out, Dendroccelum resembles in this respect 

 the earthworm Allolohophora joetida, in which, according to 

 Morgan ('97), regeneration of a head does not usually occur 

 posterior to the fifteenth segment. In later work upon this form 

 Morgan ('02) has discovered that the regeneration of the head 

 appears to be closely connected with the presence of an anterior 

 cut surface of the nerve cord, so that if two such surfaces are pre- 

 sented by removing the head and then cutting out a small piece 

 of the nerve cord a short distance posterior to the cut end, a head 

 will regenerate from each of the cut surfaces. 



In my previous paper on Leptoplana (Child, '04) I suggested 

 that the nervous stimuli in the region of a cut surface may exercise 

 either directly or indirectly some influence upon the growth of new 

 tissue from this region, and, moreover, that after removal of a 

 part they may even be increased in intensity because of the more 

 or less ineffectual attempts of the animal to perform the character- 

 istic movements. 



The facts and conclusions cited, together with many others, 

 such as the cases described by Herbst ('96a, '96b, '99) of the sub- 

 stitution of an antenna-like organ for an eye in the absence of the 

 optic ganglion from the eye-stalks of certain decapod Crustacea 



