LARVAE OF ARBACIA PUNCTULATA 357 



Vernon's measurements correspond to body-length alone, but 

 since he also determined the arm-lengths and found them prac- 

 tically unaffected by alterations of medium, the general shape 

 of his growth-curve, if hot his actual percentages, may be used 

 for comparisons. 



Vernon found in Strongylocentrotus that maximum growth 

 occurred at 950 cc. sea-water diluted to one liter; with Arbacia 

 it lies at about 900. In concentrations of less density than 

 this, the general growth-curve falls off somewhat more rapidly 

 in Strongylocentrotus than in Arbacia. In the former at about 

 875 cc. sea-water to the liter the growth rate is approximately 

 equal to that in normal, while the same is true with Arbacia at 

 about 825. Vernon was unable to secure development in water 

 of greater dilution than 150 cc. distilled water to the liter, 

 whereas Arbacia under similar treatment reaches a size above 

 the normal. Upon further dilution of the sea- water, Arbacia 

 larvae show a steady decrease in size until 750 cc. per liter is 

 reached, when, as the limit of endurance is approached, a sudden 

 drop occurs. A somewhat similar curve is produced in sea- 

 water of greater concentration, though with a less abrupt fall. 

 Vernon, however, figures a very slight decline, so that as a 

 concentration of 1150 cc. to the liter is approached, a point 

 beyond which growth is impossible, the decrease in size is ex- 

 ceedingly slight. 



In view of the great variation in growth produced by slight 

 modifications in the concentration, Vernon suggests that the 

 variation known to exist in the concentration of sea-water must 

 have some influence upon ths size of the larvae. Garrey ('15) 

 gives a table of the freezing point depressions of ocean water in 

 various localities, ranging from a = 1.093 (Dakin) at Kiel har- 

 bor, to A = 2.29 (Bottazi) at Naples. At Woods Hole the 

 concentration varies considerably in different localities and 

 under different conditions such as tides and rain-falls, as shown 

 by Garrey in an earlier paper ('04). Here he also gives a table 

 of freezing point depressions of water from various locahties in 

 that vicinity which shows variations in a ranging form a = 1.78 

 in the laboratory tap water after one extremely heavy rain, to 



JOURNAL OF MOKPHOLOGY, VOL. 30, NO. 1 



