DIFFEREXTIAL SUSCEPTIBILITY IX POLYCHETES 9 



is now e^ndent. After posterior elongation begins, the suscepti- 

 bility of the apical region is usually slightly lower than that of 

 the posterior growing region (fig. 11 j and the trochal region and 

 the dorsal part of the first body segment are the least susceptible 

 ectodermal regions. As in earlier stages, mesoderm and ento- 

 derm are less susceptible than ectoderm, entoderm least suscepti- 

 tible of all. The stomodeum, however, shows its ectodermal 

 origin in a susceptibility considerably higher than that of the 

 entoderm. Essentially the same relations persist as long as the 

 larvae Uve. 



Xereis 



The differences in susceptibility as indicated by disintegration 

 and death in different regions of the embr>^onic and larval body 

 of Xereis limbata are similar to those observed in Chaetopterus. 

 In the early stages of development the apical region is most 

 .susceptible, the basal least. As development proceeds, the 

 susceptibility of all parts increases up to the fully developed 

 larval stage (Child. '15 a. p. 414), but in the later larval period 

 there is apparently a slight decrease in susceptibility which may 

 be merely incidental to the gradual starvation of the larvae 

 which, in these forms, does not bring about extensive reduction 

 and increase in metabolic rate, as in many of the lower animals 

 (Child, '15 a, Chapter VII). but soon ends in death, as in the 

 higher forms (Child, '15 a, pp. 297-301), As soon as posterior 

 elongation begins, however, the somatic plate, and particularly 

 the posterior growing region of the somatic plate, shows a more 

 rapid increase in susceptibility than other body-regions and 

 soon becomes and remains the most susceptible region of the 

 body. The extreme posterior terminal region is less susceptible 

 than the growing region immediately anterior to it. In the 

 fuUy developed larva with three segments death begins in the 

 ectoderm of the pKDsterior growing region and progresses ante- 

 riorly over the larval segments, more rapidly on the ventral than 

 on the dorsal side. Death begins in the apical region somewhat 

 later than in the p)osterior growing region and spreads over the 

 pretrochal region, the two death gradients meeting in the region 



