128 P- B. SIVICKIS. 



the anophthalmic, and no ganglia in the acephalic form (Child and 

 McKie, 'n). In this species, as in P. dorotocephala, no other 

 head forms have been observed except secondary modifications of 

 some one of these forms in acclimation to, or recovery from, in- 

 hibiting agents (Child, '21) and inequalities or asymmetries in 

 position of eyes resulting from oblique section or other incidental 

 conditions and usually temporary. The frequency of occurrence 

 of these various forms in any lot of pieces is the head frequency 

 of the lot, and the following experiments on reconstitution are 

 chiefly concerned with the relations of head frequency to length of 

 piece, level of body, and size of animal from which pieces are 

 taken. 



The results of reconstitution in very short pieces require men- 

 tion. Forms with heads, usually normal, but without posterior 

 ends, "tailless forms" (Fig. 19) and "biaxial heads" (Fig. 20), 

 appear rarely in sixths of large animals, more frequently in 

 eighths, and their frequency increases with decreasing length of 

 piece to the limit of length at which wound closure and reconstitu- 

 tion fail to occur. Under ordinary conditions these forms are 

 much more frequent and appear in longer fractions of the body in 

 this species than in P. dorotocephala. In the latter species they 

 have never been seen in sixths or eighths, except rarely in eighths 

 from small young animals, and they are rare even in sixteenths 

 and twentieths of large animals. 



Head Frequencies in Relation to Length of Piece, Level 

 of Body, and Physiological Age of Animal. 



The data presented in this section include head frequencies of 

 fourths, sixths, eighths, and sixteenths of full-grown animals 

 11-13 mm. in length and fourths and sixths of young animals 4-6 

 mm. in length. In the series of longer pieces the death rate is 

 negligible, but in the sixths of young and the sixteenths of old 

 animals it becomes high enough to lessen considerably the value of 

 the data on head frequencies, and in pieces shorter than these it is 

 still higher, so that determination of head frequencies becomes 

 impossible in such pieces. 



i This increasing death rate with decreasing length of piece is 

 certainly to a considerable extent a consequence of increasing area 



