282 C. M. CHILD 



Between the headless pieces and the wholes various intermediate 

 forms occur. The most common type of these is shown in fig. 

 9a, where a long slender outgrowth arises after a time from the 

 anterior end, which at first was like that in fig. 7: this outgrowth 

 is highly mobile and is used by the animal in much the same manner 

 as a head. In most cases of this sort which I have observed de- 

 velopment proceeded no farther than this, even though the pieces 

 were kept for several months, but in a few cases a single median 

 eye finally appeared in the outgrowth and in one case the elongated 

 outgrowth underwent, in the course of several weeks, a gradual 

 transformation into a head with a single median eye (fig. 96). 

 In short, the development of new tissue at the anterior end may 

 show any condition between that of fig. 8 and a complete head 

 and it is possible to determine experimentally with considerable 

 exactness what stage or condition a given piece shall attain. 



In certain of my experiments it has been found desirable to 

 distinguish pieces which show a distinct anterior outgrowth of 

 new tissue (fig. 7, 9a) from those in which the new tissue simply 

 closes the wound (fig. 8). Various facts show that the outgrowth 

 represents an approach to head-formation. Such pieces are dis- 

 tinguished as ' anopthalmic ' from the strictly headless pieces. 



The larger or more vigorous headless pieces always possess a 

 well developed posterior outgrowth (figs. 7, 8, 9a), but with de- 

 creasing size and vigor the amount of new tissue produced at the 

 posterior end decreases until it becomes impossible in some 

 cases except when a pharynx is present, to distinguish the ante- 

 rior from the posterior end (figs. 10 and 11). Such pieces are 

 often ''headless" to such a degree that not merely the head, but 

 the whole prepharyngeal and pharyngeal regions are absent. It 

 is impossible to draw any sharp Une as regards appearance be- 

 tween such pieces and the fourth group. 



'Biaxial tails'. In such pieces a posterior end arises at the 

 original anterior as well as at the posterior end of the piece 

 (fig. 12). Such pieces can be recognized with more or less cer- 

 tainty by their movements and in consequence of these move- 

 ments they undergo some remarkable changes of shape, which 

 will be considered later. In P. dorotocephala these double tails 



