DIFFERENTIAL SUSCEPTIBILITY IN POLYCHETES 21 



requires a certain amount of time to penetrate and produce its 

 effect, during which time development is progressing and con- 

 ditions are changing more or less rapidly. 



The general form-modifications observed are indicated in 

 figures 24 to 47. Figures 24 to 27 show the normal development 

 at one, one and one-half, two and one-half and four and one-half 

 days respectively. Figures 28 to 37 show inhibited larval forms 

 obtained when eggs develop in KNC from a stage just preceding 

 the first cleavage. Figure 28 shows the characteristic condition 

 after three days in KNC m/50000. The pretrochal region is 

 small and the post-trochal region large, as compared with cor- 

 responding normal stages (fig. 25), setae are absent, and there is 

 no definite posterior elongation. In this concentration death 

 usually occurs about the fourth day without any marked further 

 advance in development. Figures 29 to 37 show larvae after 

 four to six days in KNC 7?i/ 100000, beginning at the first cleav- 

 age. In all cases the development is of course retarded, but 

 the pretrochal region is relatively smaller and the post-trochal 

 region relatively larger than in normal forms. In more advanced 

 stages this differential effect of the KNC becomes still more 

 marked, but at the same time the later development of the poste- 

 rior growing region and the development of the third body-seg- 

 ment are more or less completely inhibited (figs. 34 to 36, five 

 days) . The development of the setae is also markedly inhibited 

 in many cases, some individuals having only a few setae (figs. 

 31, 32, 34, 35) while others are entirely without setae and remain 

 so (figs. 33, 36). Figure 36 shows a six-day old KNC larva 

 from the same series. Here the head is still relatively smaller 

 and the body region relatively larger than normal (fig. 27), 

 though evidently retarded in development. 



These larval forms, like those of Chaetopterus inhibited from 

 the beginning of development, show some degree of micro- 

 cephaly and in later stages an inhibition of the posterior growing 

 region. It is possible that forms like figure 37, which are found 

 after six days or more in KNC, represent some slight degree of 

 differential acclimation in the posterior growing region, for this 

 region apparently shows an increase in rapidity of development 



