LARVAE OF ARBACIA PUNCTULATA 403 



Figure 117 is from the control, 118 from solution d, 119 from e, 

 120 and 121 from/. In the control (fig. 117) all the specimens 

 examined were strictly bilateral and the same was true in h. 

 In c a very few, not more than one or two in each hundred meas- 

 ured had three spicules present at the age of 24 hours, but in 

 solution d (fig. 118) this phenomenon was rather more common 

 and appeared in about ten per cent of the specimens. In solu- 

 tion e (fig. 119), four spicules were developed in a still larger 

 percentage, while some possessed three and the remainder, two. 

 In solution /, four seemed to be the predominating number 

 among those in which radial symmetry was produced (fig. 120), 

 while many possessed three and a few, five (fig. 121). In solu- 

 tions of increasing CO2 content, there was very little inhibition 

 of growth, as may be seen from the figures, and in each specimen 

 all the spicules were apparently equal and normally shaped. 



Figures 122, 123 and 124 are from solutions a, 6 and c re- 

 spectively at the age of 48 hours. The skeletons are approxi- 

 mately of the same size, but those in the CO2 cultures are heavier 

 and slightly less regular. This change is still more marked in 

 solution d (fig. 125) and in addition a small accessory spicule is 

 present in the right side. These accessory spicules were much 

 less frequent in the specimens 48 hours old than in those 24, 

 and in many cases were still smaller than at the early period, 

 indicating that resorption was taking place. Figure 126 illus- 

 trates another specimen from the same culture. Its skeleton 

 is somewhat more regular and is not so heavy as that represented 

 in figure 125, but the accessory spicule which lies at the left side 

 has enlarged and possesses the main parts characteristic of nor- 

 mal skeletons : body-rod, dorso-ventral connective, anal and oral 

 arm-rods. In the right side, compensatory structures have de- 

 veloped in the form of accessory rods on the lateral connectives. 

 In the side in which the accessory skeleton is present, the arm- 

 rod is more slender, and instead of the usual three, is composed 

 of two single rods in a less advanced stage of fusion. 



Figures 127 and 128 are from solution e. The specimen repre- 

 sented in 127 was smaller but the skeleton was heavier and more 

 irregular. In 128, four spicules are present but although irregu- 



