350 



Abigail C. Dimon. 



tion at the levels of these experiments, the cases in which they ex- 

 ist are classified in the table as a separate group under Head A. 

 Cases where the brain lies anterior and even ventral to the level 

 of the digestive tract; where the nerve cord ends without form- 

 ing a brain; or where there is no mouth invagination, though the 

 brain is well developed, are classified as Head B. The heads of 

 group B look less like a normal head than those of group A, and 

 yet are very evidently to be classified as heads rather than as tails. 

 The distinctive features indicating a heteromorphic tail are the 

 formation of a number of segments, the opening of the digestive 

 tract to the exterior through a new anus, and the ending of the 

 nerve cord ventrally, without a brain. Tails possessing these fea- 

 tures are put in group A, while those in which any of these features 

 are absent, are put in group B/ 



In all, one hundred and seventeen worms were examined, with 

 the results given in the table. The number of the segment given 

 at the head of each column locates the level at which the worm was 

 cut in two, and both the actual number of worms and the percent- 

 ages are given under each class. 



iJn only twenty-four cases out of one hundred and seventeen did the old di- 

 gestive tract open to the exterior through the new mouth or anus. This occurred 

 ten times in head regeneration, ten times in heteromorphic tail regeneration, 

 and four times in cases classified as uncei^tain. Seven of the twenty-four cases 

 occurred when the worm was cut in two in front of the sixteenth segment, and 

 the other seventeen when it was cut behind the eighteenth segment. 



