DIFFERENTIAL SUSCEPTIBILITY IN POLYCHETES 17 



Chaetopterus 



The usual course of larval development in sea water at ordinary 

 temperature is indicated in figures 12 to 14, A being the lateral, 

 B, the ventral aspect of the larva. Figure 12 is a 12 hour stage, 

 figure 13, A, B, twenty-four hours and figure 14 A, B, forty- 

 eight hours. 



Figures 15 to 17 show three-day larvae, the most advanced 

 stages attained by eggs placed in KNC m/ 100000 thirty minutes 

 after fertilization and undergoing their whole development in 

 this solution. As compared with normal larvae of forty-eight 

 hours (fig. 14), the three-day larvae of figures 15 and 16 are 

 smaller, shorter and very evidently less advanced in develop- 

 ment. The head-region is distinctl*'' inhibited, the region of the 

 first two larval segments is almost as well developed as in the 

 normal animal and the posterior region again shows marked 

 inhibition. 



The larva of figure 17 is a form characteristic of both KNC 

 and HCl cultures, sometimes constituting 25 to 50 per cent of 

 the living individuals. As regards the stage of development 

 attained, the larva is evidently inhibited, like those of figures 

 15 and 16, but the body-region is much enlarged or distended. 

 All degrees of such enlargement, ranging from forms like figures 

 15 and 16 to those like figure 17 and even still more enlarged 

 forms, occur. As regards general activity and behavior these 

 larvae do not differ from the others and do not die earlier than 

 the others. Since they are not merely an occasional but a 

 characteristic feature of the inhibition cultures, I am inclined 

 at present to regard them as a result of differential inhibition 

 and to account for them as follows : they appear to be due to an 

 excess of fluid in the body cavity and perhaps also in the enteric 

 cavity. The coelomic fluid at least is, to some extent, a metabolic 

 product, for instance directly or indirectly the result of cellular 

 activity. If the cells chiefly concerned in its production possess 

 a relatively low susceptibility, their activity in these differentially 

 inhibited larvae will be relatively greater and may occur to a 

 greater extent at the expense of other parts than in normal 



JOURNAL OF M9RPHOLOGY, VOL. 30, NO. 1 



