134 CM, CHILD 



was this difference which led to the recognition of the existence of 

 posterior zooids (Child, '06, '09, '11 c, '11 f). 



Under certain experimental conditions the axial factor in rela- 

 tion to the anterior and posterior zooids appears even more clearly 

 than under the usual conditions. Here, as in the experiments 

 described above, various conditions, e.g., different temperatures, 

 alcohol and other anesthetics, KCN, etc., may be used to bring 

 out these differences. Such experiments show beyond a doubt 

 that the zooids are marked off more or less clearly by differences 

 in dynamic conditions along the main axis. But the actual 

 results as regards the regulation of the pieces differ very widely, in 

 certain cases diametrically, according to the manner in which the 

 reagent or condition is used in the experiment. At present, 

 however, it is desired merely to show that the posterior zooids 

 are distinguishable from the anterior by means of this dynamic 

 axial factor. The problem involved in the different effects of a 

 given agent under different conditions will be taken up later. 



Only a few of my experimental series which bear upon this point 

 are given below : KCN and temperature series are selected because 

 they demonstrate the point clearly and have a much lower mor- 

 tality than the series with anesthetics. In all cases the series are 

 primarily concerned with the dynamics of regulation rather than 

 with the point considered here which is merely an incidental result. 



In order to avoid repetition it may be stated that in all the 

 series given below worms 18 to 20 mm. in length were used in 

 these worms the anterior zooid usually included about half the 

 total length and the posterior half consists of either two or three 

 well-defined zooids. In figure 46 the region in which the anterior 

 end of the second zooid occurs is indicated by the shaded area at 

 XX. The position of the boundary between the two zooids varies 

 in different individuals and the length of the shaded area in the 

 figure represents approximately the limits of variation, while the 

 frequency of its occurrence at different levels within this limit is 

 roughly indicated by the density of the shading. 



Near the posterior end of the body in these worms a third zooid 

 or a series of short zooids (Child, '11 f) exists and the region of the 

 boundary between it and the second zooid or its descendants is 



