78 C. M. CHILD 



although wider than normal, is not as great as that between the 

 arms. In these cases the effect of differential acclimation has 

 been extreme in the apical and anterior regions, but relatively 

 less in more basal and posterior parts. Figures 31 to 36 show the 

 final stages of development attained in the more extreme degrees 

 of differential acclimation to acids. Figure 31 A is the basal 

 and figure 31 B the lateral aspect of a larva with large oral lobe, 

 small body and no arms. The body is short and broad and very 

 wide angled. Evidently apical and anterior regions are rela- 

 tively over-developed, basal and posterior relatively under- 

 developed. In figure 32 A (basal) and B (lateral), these modi- 

 fications are more extreme. The oral lobe is relatively longer, 

 the body shorter and broader, and the angle of divergence wider. 

 Figure 33 shows a still more extreme degree of modification, A 

 being the basal, B the lateral and C the anterior aspect, and 

 figure 34 shows the most extreme modification possible in this 

 direction, A, B, and C being, as before, basal, lateral and anterior 

 aspects. Here the oral lobe is larger than the body, and the 

 skeletal rods are united to form a single straight rod lying trans- 

 versely, and passing anterior, instead of posterior, to the anus. 

 The angle of divergence between the rods is here 180°. These 

 forms in figures 31 to 34 differ from those in figures 29 and 30 

 in that the angles of divergence of the skeletal rods are uniform 

 throughout, instead of being wider anteriorly than posteriorly. 

 This means that in figures 29 and 30 the differential effect of 

 acclimation is more extreme anteriorly than posteriorly, while in 

 figures 31 to 34 it has extended far enough posteriorly to deter- 

 mine a uniform angle of divergence of the whole skeleton, in- 

 stead of a wider angle anteriorly than posteriorly. In figures 35 

 and 36, A, B, C, forms similar in outline and axial relations, but 

 without skeleton, are shown. 



It is evident that all these forms are the result of relative dif- 

 ferential accelerations along the axes. Acclimation has made 

 the slope of the axial gradients steeper; i.e., the regions of highest 

 metabolic rate in each gradient have now a relatively much higher 

 rate, and the regions of lowest rate a relatively much lower rate 

 then under normal conditions. Consequently the greatest de- 



