76 C. M. CHILD 



would if no differential acclimation had occurred. In such cases 

 the form is not an adequate criterion, though increase in motor 

 activity and rate of development may indicate that some degree 

 of acclimation is taking place. But when the axial relations of 

 differential development are the opposite of those resulting 

 from differential inhibition, the form is sharply distinguishable 

 from the form produced by differential inhibition, and it is with 

 such forms that we are at present concerned. 



Where the direct inhibiting action of the agent is not very 

 great, differential acclimation does not produce any extreme 

 modifications of form, but appears merely as a change in the 

 proportions and angles of divergence of parts of the plutues. 

 Figure 23 A and B, shows a differentially acclimated pluteus in 

 basal and lateral aspects. A comparison of this figure with 

 figures 5 A, B, which represent the usual form of plutei raised 

 in pure sea water shows that, in the acclimated form, the oral 

 lobe, i.e., the apical region, is longer and broader, particularly 

 at its apical end, the body is shorter and less slender, and the 

 angles of divergence between the two anal arms, between anal 

 arms and oral lobe, and between the short arms on the oral lobe 

 is greater than in the sea water forms. Moreover, a comparison 

 of figure 23 with figures 6 A, B, which show results of slight 

 direct inhibition, makes the opposite character of differential 

 inhibitory and differential acclimatory changes strikingly evident. 



Not infrequently in these wide angled forms, the angle be- 

 tween the anal arms is somewhat wider than between the poster- 

 ior skeletal rods, as in figure 24. This indicates a greater de- 

 gree of disproportion in development between the anterior region 

 and the more posterior levels than in figure 23, i.e., breadth of 

 the anterior region is increased out of proportion to that of other 

 levels of the antero-posterior axis, and the angle between the 

 arms is therefore wider than that between the more posterior 

 portions of the skeleton. 



In KCN and NH 4 OH, where acclimation is relatively slight 

 and occurs so slowly that development is usually completed be- 

 fore the differential effects of acclimation become marked, even 

 the slight changes of form shown in figure 23 do not usually 



