134 p - B - sivickis. 



Comparison of the curves for P. lata with those for P. doroto- 

 cephala in Figs. 21 and 22 shows that the changes in head fre- 

 quency with length of piece and level of body are in general of 

 the same sort, but that their range is very much greater in P. 

 dorotocephala than in P. lata. In full-grown animals (Fig. 21) 

 the head frequencies of fourths of P. dorotocephala are somewhat 

 higher than, those of sixths about equal to, and those of eighths 

 and sixteenths far lower than the head frequencies of correspond- 

 ing pieces of P. lata from the posterior regions of the first zooid, 

 while at anterior and posterior ends of the body the differences 

 between the two species are slight. In the young animals (Fig. 

 22) fourths and sixths from the posterior region of the first zooid 

 of P. dorotocephala are far below fourths and sixths of P. lata 

 from the same region, while at the anterior end of the body the 

 differences are much less, though greater than in old animals. 



The graph, Fig. 23, is a comparison of the curves of fourths and 

 sixths of full-grown and young individuals of P. lata. This graph 

 shows that the head frequencies in young animals are slightly lower 

 than in corresponding pieces of old animals, but this age differ- 

 ence is much less than in P. dorotocephala. Comparison of Figs. 

 21 and 22 shows that the curves of fourths and sixths of young 

 P. dorotocephala are much steeper and fall much lower than those 

 of fourths and sixths of old animals. 



Tables I. and II. show the frequencies of biaxial heads, but not 

 of tailless forms. Unfortunately tailless forms were not recorded 

 as such in the earlier experiments. It may be stated, however, 

 that they do not appear in the longer pieces, that their frequency 

 increases with decreasing length of piece, and that, so far as data 

 are at hand, they show no definite relation to level of body. As 

 regards the frequencies of biaxial heads, Tables I. and II. show 

 that they do not occur in the longer pieces, that their frequency 

 increases with decreasing length of piece, and is apparently some- 

 what greater near the level at which fission occurs than elsewhere. 



Experimental Alteration of Head Frequency. 



In P. dorotocephala the head frequencies have been altered ex- 

 perimentally by many different chemical and physical factors 

 (Behre, '18; Buchanan, '22; Child, '16, '20a). Moreover, it has 



