I32 P. B. SIVICKIS. 



In the graph, Fig. 21, the data for full-grown animals given in 

 Table I. are plotted, together with head- frequency curves from 

 similar experiments with P. dorotocephala. In Fig. 22 the data 

 for young worms from Table II. are graphed in comparison with 

 corresponding data of P. dorotocephala. 2 As regards region of 

 body, the data show, both for full-grown and for young animals, 

 that head frequency decreases from the most anterior level of sec- 

 tion posteriorly and then increases again with approach to the level 

 of the posteriod zooid, until at the most posterior level of section 

 it is as high as at the most anterior level, except when lowered by 

 early deaths. 



Second, as regards length of piece, the data show for old ani- 

 mals that decrease in head frequency from anterior to posterior 

 region of the first zooid and increase at levels further posterior 

 becomes greater as length of piece becomes less. The shorter the 

 pieces, the steeper the downward and upward slopes of the curves 

 in Fig. 21. At the most anterior and most posterior levels of the 

 body fourths, sixths, and eighths are practically alike in head fre- 

 quency, sixteenths somewhat lowes anteriorly, but the differences 

 in level of the lowest points of these curves is considerable. The 

 irregularities in the curve of sixteenths are due to the differences 

 in length of the pieces, the variations being, of course, relatively 

 much greater in these very short pieces than in longer pieces. In 

 the young worms (Fig. 22) the differences in steepness in relation 

 to length of piece do not appear in the only data available, those 

 for fourths and sixths, but the sixths show a somewhat lower head 

 frequency than the fourths, except at the posterior end, where it is 

 the same in both. 3 



2 The curves of head frequency of P. dorotocephala are plotted from data 

 obtained from various sources : fourths, sixths and eighths of old animals 

 and fourths and sixths of young animals from Child Cub, '16, '20a), one 

 series of eighths of old animals from Miss M. A. Hinrichs and a complete 

 series of my own for both old and young. All of these data are in general 

 agreement as regards relation of head frequency to length of piece, level of 

 body and physiological age. 



s It may be noted that the data for young worms are made up in part from 

 animals raised in the laboratory from eggs and in part from animals of the 

 same size similarly raised from cut pieces. The pieces of animals from eggs 

 showed a somewhat higher death rate and therefore a somewhat lower head 

 frequency than those from cut pieces, but the differences were not great. 



