Regeneration of Heteromorphic Tails. 389 



time. The possibility that all the anterior pointed structures 

 may be only undeveloped heads must therefore be given seri- 

 ous consideration. That they are not such in many cases is 

 shown, I think, by the following facts. In the first place the 

 movement of a piece in which the anterior head is undeveloped 

 is very different from that of the two-tailed pieces, and reveals the 

 nature of the new part; for, while the former crawls forward as do 

 the pieces when first cut from the worm and as do those that 

 develop an anterior head, the two-tailed pieces remain fixed in one 

 place in the dish, and, if disturbed, fail to move in any definite 

 direction. This is what we should anticipate if two tails were 

 present working in opposite directions. 



In the second place an orthomorphic pharynx appears as a 

 rule when the head is delayed in its development while none such 

 appears in the two-tailed pieces. In the third place the peculiar 

 motion of the anterior end when it is irritated is similar to that of 

 a tail and not like that of a head. Finally, the development in 

 one case to be mentioned below of a two-tailed piece with pharynx 

 in each tail shows, beyond a doubt, the possibility of the develop- 

 ment of a heteromorphic tail in these worms. 



After the short cross-pieces have been cut off for some time it 

 is difficult to distinguish the anterior from the posterior end and to 

 know which is the anterior heteromorphic and which is the normal 

 posterior orthomorphic tail. In order to distinguish these apart, 

 I cut off pieces obliquely at one end; in one set the anterior end 

 being the oblique one. Fig. 16, in the others the posterior, Fig. 18. 

 This necessitated increasing somewhat the length of the pieces and 

 brought about in consequence an increase in the number of the 

 pieces that regenerated a head at the anterior end. The record of 

 one set of experiments of this sort is given here. 



On May 7, twelve short cross-pieces were cut off just behind the 

 pharynx. The anterior end of each was oblique. On May 20 

 there were alive three two-tailed pieces; the rest having died. 



Twelve short tail-ends regenerated new tissue at the anterior 

 end which in two cases at least appeared to be tails. 



In another series twelve short cross-pieces were cut off behind 

 the pharynx. The anterior end was square and the posterior end 



