PROCESS OF REGENERATION 141 



5. Regeneration in the tuhificids 



In these forms, the inhibition of head formation has proceeded 

 to such an extent that all pieces back of a certain anterior level 

 remain headless. By removing definite numbers of anterior 

 segments, one may determine the level at which head formation 

 ceases. In the case of Tubifex, five, ten, fifteen, and twenty 

 anterior segments were removed, with the following results. 

 At the level of the fifth segment, a normal head is usually regener- 

 ated. The normal head of Tubifex consists of five segments, 

 four setigerous, and contains mouth, brain, pharynx, etc. When 

 ten anterior segments are removed, the result is the same. When 

 fifteen are cut off, some normal heads are regenerated, but most 

 of the pieces give rise to inhibited heads, and some remain head- 

 less. I find in my notes unfortunately, no description of the 

 appearance of these inhibited heads, nor any statement as to 

 whether various types could be distinguished. At the level of 

 the twentieth segment, all pieces remain headless. Between the 

 fifteenth and the twentieth segments, therefore, head formation 

 ceases rather abruptly, although inhibited heads occur as stages 

 between the normal and the acephalic condition. These results 

 agree with those of European investigators working on the same 

 species, Tubifex tubifex, except that they found head formation 

 to cease at a more anterior level — tenth to twelfth segment 

 (Abel '02, Haase '98). The regeneration of the head is slow in 

 Tubifex, and the greater the number of anterior segments re- 

 moved, the more slowly does regeneration take place. 



In Limnodrilus claperedianus, the capacity for head formation 

 is even more limited than in Tubifex. As this species has al- 

 ready been thoroughly investigated by Krecker ('10), it was not 

 necessary for me to perform any very extensive experiments. 

 The head of Limnodrilus consists of four segments, but two is the 

 maximum number regenerated; of these, the first bears prosto- 

 niium, mouth and brain, all of normal appearance, and the second 

 is setigerous. From this condition, one finds the following 

 gradations to the acephalic state; regeneration of one segment 

 only with normal prostomium, mouth, and brain, reduction of 



