DIFFERENTIAL SUSCEPTIBILITY IN POLYCHETES 19 



centrations of HCl than of KNC must, of course, be used and, 

 even in concentrations high enough to retard the earher stages 

 of development and give differential inhibitory effects, some 

 degree of differential acclimation may occur in the later stages. 

 The differences in action between KNC and HCl are, in general, 

 the same for the polychete as for the sea urchin (Child '16 c). 

 The larval forms produced by development in HCl m/2500 

 from thirty minutes after fertilization (figs. 20, 21) are not dis- 

 tinguishable in any characteristic way from the KNC forms 

 (figs. 15, 16), and the distended forms (fig. 17) occur as frequently 

 in HCl as in KNC. In some lots development goes no farther 

 in HCl m/2500, but in others some degree of differential acclima- 

 tion occurs, giving forms like figure 22 in which the development 

 of head region and posterior end is progressing slowly and the 

 body is becoming more elongated as in the normal animal. 



When development is allowed to proceed for twenty-four 

 hours in water and the younger trochophores are then placed in 

 KNC or HCl, the head and anterior body region are relatively 

 large, and the inhibition is limited almost entirely to the posterior 

 body-region (fig. 23) . At twenty-four hours the posterior region 

 has become the most active and so the most susceptible region 

 of the young larva. Exposure to KNC and HCl after eight to 

 twelve hours development in water gives results intermediate 

 between those of exposure from the beginning of development 

 and exposure after twenty-four hours development. The head 

 region is relatively smaller than in figure 23, but not so small as 

 in figures 15 to 21. 



As regards the limits of concentration of agents used, KNC 

 m/ 100000 for the whole period of development usually gives a 

 high percentage of differential inhibitions and a considerable 

 mortality within three days. Some lots of eggs will develop 

 in KNC m/50000, others die after two days with greatly retarded 

 development. Higher concentrations can be used only for tem- 

 porary exposure. KNC m/10000 can be used for periods up to 

 twelve to fourteen hours at the beginning of development, but 

 this length of time will kill larvae twenty-four hours or more old. 

 HCl m/5000 for the whole developmental period does not pro- 



