LARVAE OF ARBACIA PUNCTULATA 411 



doch noch bilaterale Larven von annahernd Plutousform aus den 

 urspriingiich mehr radiar aussehenden Larven hervorgehen." 



This is what occurs in Arbacia. During the first 24 hours, 

 all the skeletal centers develop equally (figs. 118 to 121) and 

 in very strong solutions the radial symmetry is maintained until 

 later periods (figs. 128, 131 and 134, e, f and g, 48 hours). 

 Either these larvae fail to develop further, or else, as Herbst 

 describes, two spicules which most closely approximate the nor- 

 mal in position, elongate and give the animal a bilateral form. 

 The other skeletal centers may remain without undergoing 

 any change for some time, (fig. 133 g, 48 hours), but subsequently 

 a shifting occurs, so that the centers come to lie upon one side 

 or other of the larva. This is especially true when the third 

 spicule ultimately develops into a secondary skeleton (fig. 126 d, 

 48 hours, etc.), and generally it is fused to one side of the primary 

 skeleton (figs. 137 /, 72 hours and 140 g, 72 hours). In figure 

 141 /, 96 hours, an accessory skeleton has remained at each side, 

 but in 142 g, 96 hours, both have shifted to one side and fused 

 with the primary skeleton. In no instance did radial symmetry 

 obtain in so late a stage as is shown by Herbst ('03), in his 

 figure 13. When a third arm appears, as in figure 137 /, 72 

 hours, it \s more or less closely applied to one of the primary. 



Addition of CO2 to the sea-water does not cause any tendency 

 toward the appearance of those characteristics specific to the 

 carbonate solutions. Since addition of CO2 produces increase 

 of CO3 and HCO3 ions, the absence of any tendency toward the 

 carbonate effect when COo is added, gives further evidence for 

 the conclusion reached previously, that those modifications are 

 caused by the molecular calcium and magnesium carbonates, 

 rather than by the CO3 or HCO3 ions. 



In another culture run parallel to this experiment, the fertili- 

 zation was made m normal sea-water and the medium left un- 

 changed for the following five days, in order to determine 

 whether the modifications were similar to those of the carbon di- 

 oxide solutions. It was not to be expected that the growth- 

 curve would be like those of the others since in this instance the 

 carbon dioxide would appear in the medium as a gradual ac- 



