REGENEKATION OF POLYCLADS 55 



worm broke through and left two finger-Uke projections. In 

 these specimens there was never any regeneration beyond the 

 heahng over of the raw edges and the projecting fingers remained 

 indefinitel.y. \"\Tien the rim of the hole remained intact, new 

 tissue filled in the space within two days. After a week one could 

 see the parts of the digestive tract joining up in the new tissue 

 so as to form continuous tubes, and at the end of three weeks 

 pigment spots showed that eyes had been regenerated. These 

 eye spots showed a tendency to be grouped into cephalic and 

 tentacle eyes as in the normal individual even in the absence of 

 the brain (fig. 9). "\\Tien the posterior end of such a brainless 

 specimen was removed, a new tail was regenerated in the usual 

 manner. 



The results of these experiments bear out Miss Lloyd's idea 

 that "the mechanism for the restoration of the tail belongs to the 

 body as a whole, while that for restoring the head is an entirely 

 independent one, which may or may not be localized in some part 

 of the body, notably the anterior end." In these three species 

 of polyclad worms this localization of the mechanism for the 

 regeneration of anterior parts is undoubtedly in the cephalic 

 ganglia. The exact nature of this relation of the central nervous 

 system to the powers of regeneration still remains unsolved, and 

 the merits of the various suggestions that it may be a matter of 

 enzymes, flow of organ-forming substances, hormones, or dif- 

 ferences in axial gradients have yet to be proved. 



SUMMARY 



Planocera californica, Phylloplana littoricola, and Leptoplana 

 saxicola follow the rule for polyclad regeneration. They are able 

 to restore missing parts, provided the cephalic ganglia are intact. 

 If these are injured, new nervous tissue is not added to restore 

 the brain to its original size, and if they are entirely removed, 

 regeneration cannot take place anteriorly, though it may do so 

 posteriorly. If a portion of the brain remains, a certain amount 

 of anterior regeneration takes place and eyes are regenerated, 

 but not enough material is added to restore the original form, 

 and the new eyes never reach the size of the old ones. 



