DOUBLE STRUCTURE IN PLANARIA LUGUBRIS. 139 



explanation of the presence of a heteromorphic head. In order 

 to see if any internal condition is present in these double forms 

 that may account for the results, the two preceding pieces from 

 the posterior end, as well as two other similar pieces, were cut into 

 sections. In three of these no pharynx was present, but parts ot 

 the old reproductive organs were present in the middle of the 

 piece ; in the fourth a pharynx was present, but the pharyngeal 

 chamber did not open to the exterior. There was nothing in the 

 presence of the digestive tract to give a clue as to the cause of the 

 development of a heteromorphic head. It is true that the branches 

 of the digestive tract were united behind, but this in itself furnishes 

 no proof that such a union can account for the result. The sec- 

 tions show that a brain, nerve-cords, and eyes are present in both 

 heads. Whatever the factors are that determine the result, the 

 fact that the heteromorphic head appears only in short pieces 

 will probably also have to be taken into account, for it is veiy 

 improbable that a heteromorphic head would appear if whole 

 worms were cut through in this region. 



o o 



BRYN MAWR, PA., May 25, 1902. 



