DIFFERENTIAL SUSCEPTIBILITY IN POLYCHETES 23 



larger, indicating that the growing region has recovered its 

 activity to some extent in the later stages. The head remains 

 relatively large and is more advanced than in the six-day stage 

 in KNC from the first cleavage (fig. 37) but setae are still entirely 

 absent. Some individuals of the same type show a few setae. 



A few experiments on temporary exposure to much higher 

 concentrations of KNC were performed, but the material was not 

 sufficient for any large number of these. In these experiments 

 young moving larvae twelve hours after fertilization were placed 

 in KNC m/1000 and returned to sea water after four, five, six 

 and one-half and eight hours. The four-hour lot produced forms 

 ranging from practically normal to slightly megacephalic, with 

 slightly inhibited posterior body-region and few or no setae. 

 The five-hour lot produced forms like figures 43 to 47, mega- 

 cephalic, with extreme posterior inhibition and entirely without 

 setae. In many of these, as in figures 45 to 47, the posterior 

 growing region develops only as a small single or bilobed out- 

 growth, the body segments never differentiate visibly, and in 

 many cases bilaterality does not appear in the segmental region. 

 Of the six-and-a-half hour lot 20 to 25 per cent died wholly or in 

 part within two days and 60 to 75 per cent within four days, but 

 a few individuals produced forms like figures 43 to 47. Of the 

 eight hour lot 80 to 90 per cent died wholly or in part within two 

 days. A few individuals lived to four days, producing forms 

 like figures 44 to 47. 



With a few lots of eggs HCl was used as inhibiting agent. In 

 HCl m/ 10000 or below, development is practically normal. In 

 HCl m/5000 the results are in general very similar to those ob- 

 tained with KNC m/1 00000, but with a higher percentage of 

 deaths within the first three days. In HCl 7?i/2500 most of the 

 larvae died within two or three days without developing beyond 

 the early trochophore stage. 



As regards particular regions and organs, it is evident from the 

 figures that the development of the setae, segmental cirri and 

 tentacles may be completely inhibited, even when the general 

 developmental modifications are not extreme. Evidently these 

 organs in their early stages are regions of high metabolic activity 



