112 LIBBIE H. HYMAN 



experiments; as examples may be mentioned the sensory auricles 

 of Planaria, and the lower lip of the oligochaetes. 



Summarizing these results, it is shown that in Aeolosoma the 

 primary gradient is present, i.e., the rate of metabolism — as 

 measured by degree of susceptibility to cyanide — is highest at 

 the head, and decreases along the antero-posterior axis. This 

 gradient is present in the zooid from the very beginning of its 

 existence, and continues during its differentiation, and after it 

 has separated. Its rate of metabolism continually increases 

 during this period, but after it has separated and begun to form 

 zooids of Jts own, the rate falls, and may eventually be exceeded 

 by that of its growing posterior end. 



3. The gradient of the naids 



The primary gradient which exists in Aeolosoma is not re- 

 tained in any other oligochaetes which I have examined except 

 in the zooid stages. I have already spoken in the case of Aeo- 

 losoma of the rise in susceptibility at the posterior end owing to 

 the formation of new segments there. Aeolosoma, however, 

 gives rise to zooids so rapidly that this growing posterior region 

 does not attain to any considerable size because it is always being 

 cut off by zooid formation. But in other Oligochaetes, especially 

 those which do not reproduce asexually at all, this posterior growth 

 is extensive, and has a marked effect on the axial gradient. 



Among, the naids, I have worked for the most part with Dero 

 limosa. A concentration of KCN of j^ or -^ was used. In 

 individuals without fission planes the process of disintegration 

 is as follows. Disintegration begins at the anterior end, involv- 

 ing the tip of the prostomium and the sensory region about the 

 mouth first, and passes posteriorly along the axis; after it has 

 progressed some distance, which varies with individuals, disinte- 

 gration begins at the posterior end and proceeds forwards; the 

 two waves of disintegration meet about the middle or behind 

 the middle of the worm, the exact point also varying with 

 individuals. 



