LARVAE OF ARBACIA PUNCTULATA 



375 



The effects upon the skeletons caused by subjection to the 

 carbonate solutions were very different, in all respects except 

 size, from those produced by the increased hydroxyl concentra- 

 tion. In the latter there was no appreciable tendency toward 

 a thickening of the skeletal structures nor toward a reduc- 

 tion of parts, but in the carbonate solutions these were the 

 most prominent and characteristic changes. Figure 81 repre- 

 sents one of the control specimens (Experiment 9), 48 hours 

 old and figure 82, one from c at the same age. All the structures 

 are heavier, especially the rods of the oral arms and the body- 



Figs. 81 to 87 Experiment 9 NaHCOg. 



Fig. 81 9a, 48 hrs. Control. 



Fig. 82 9c, 48 hrs. Structures heavier, especially oral arms and body-rods. 



rods. There has been a precocious closing of the interspaces 

 in the anal arms and the spines are reduced in number. A 

 fusion of the skeletal elements often occurs early, accompanied 

 by decided thickenings at the points of contact. In other 

 instances, even at a late period, the individual elements remain 

 distinct. The ventral body-branches are frequently present as 

 thick, fused, club-shaped masses giving httle or no indication of 

 separate processes (fig. 83, /, 72 hours). At other times, es- 

 pecially in the stronger solutions, no ventral body-branches are 

 present (fig. 84, g, 48 hours) . Figures 85, 86, and 87 are taken from 

 solution / at the age of 72 hours, and depict the typical modifica- 

 tions. Figure 85 represents one nearly resembUng the normal 



