DIFFERENTIAL SUSCEPTIBILITY IN POLYCHETES 25 



differential inhibition occurs (fig. 53), but the percentage of the 

 more extreme forms is small and most animals complete meta- 

 morphosis, while in m/ 10000 nearly all individuals show more 

 or less differential inhibition, and the majority die before complet- 

 ing metamorphosis. After four days in KNC m/10000 from the 

 beginning of elongation the alteration in the larval form is 

 present but slight, the posterior region being relatively small 

 (fig. 51) as compared with the corresponding normal stage (fig. 

 48). After six days (fig. 52) the head-region is distinctly larger 

 and the posterior region smaller than the normal (fig. 49) and 

 setae are usually entirely absent. Very similar forms occur, 

 though less frequently, in m/25000 and usually show setae on 

 one or more segments (fig. 53) . Figures 54 and 55 show similar 

 forms after eight days in KNC ??«/ 10000, the latter being a case 

 in which the development of the paratroch is completely in- 

 hibited and bilaterality of form is almost entirely absent from 

 the segmental region. 



Forms like the figures died in my experiments without com- 

 pleting metamorphosis, but individuals which are less inhibited 

 may develop into worms like figure 56, which shows relative 

 inhibition of the posterior segmental and growing region as 

 compared with the normal worm (fig. 50). The large trans- 

 verse diameter of the body in such forms is apparently due to 

 distension of the coelom by fluid, a condition somewhat like that 

 in Chaetopterus (p. 17), but less extreme. 



A few experiments with HCl, beginning at the same stage, gave 

 similar but less extreme differential results, and more or less 

 differential acclimation usually occurred in later stages in con- 

 centrations which permitted the completion of metamorphosis. 

 As regards concentrations, HCl m/10000 has no appreciable 

 effect, m/5000 retards development slightly, but gives no marked 

 differential effect, m/2500 gives differential inhibitions of the 

 same sort, but less extreme than figures 52 to 55, and the 

 posterior growing region shows acclimation and increase in growth 

 rate in the later stages; and, finally, m/1000 kills within three 

 or four days with but little advance in development. 



