WATER RELATIONS OF OTHER SPECIES 



145 



swell in diluted sea water or shrink in concentrated sea water. On 

 subsequent return to normal sea water (fig. 91), mean rates of 

 intake and of output are computed from successive measurements 

 of diameter (d). The rates of exchange in this series turn out to be 

 proportional to the differences of osmotic pressure between egg 

 and medium. However that may be, for equal differences of it, 

 intake is slightly sloiver than output, at all temperatures from 12° 

 to 24° C. 



In this spherical organism the exchanges are probably propor- 

 tional to surface area (nd^) and to the gradient of osmotic pressure, 

 according to the equation: SW/At = /iV"d^(Pi -Pe). The coeffi- 



a 



1500 



cpIOOO 



^ I 



t lS 500 



0) 



+» 

 o 



Q. ^ 



-50 +50 +100 +150 



Water Load 



Fig. 91. Eate of net water exchange (% of Bo/hour) in relation to total water 

 load (% of Bo). Unfertilized egg of the sea urchin Arhacia punctulata in normal sea 

 water at 19° C. Eate is computed as though it prevailed during the first 1/60 hour; 

 in 6 eggs transferred at zero time from each of 5 dilutions of sea water (80 to 40 per 

 cent). Eates of gain in recovery from negative loads were not reported but are believed 

 to be about 20 per cent less than rates of loss at the same positive water loads. Data 

 of Lucke and McCutcheon ('27). 



cient h represents permeability (Lucke et al., '31). When li is con- 

 stant and turnover is nil, the net equilibration diagram looks much 

 like that for Phascolosoma. 



The eggs of half a dozen other species of echinoderms have been 

 studied in an analogous manner (Leitch, '31, '36; Fukuda, '35) in 

 limited respects. For the most part, water exchanges have been 

 measured when eggs are transferred from sea water into various 

 dilutions and concentrations. By analogy with Arbacia it might be 

 supposed that exchanges in recovery depend only on the gradients 

 of osmotic pressure that prevail. The data at hand do not en- 

 courage this assumption, since significant differences in h appear 

 within the same species at transfer into 40, 50 and 60 per cent sea 



