WATER EELATIONS OF OTHER SPECIES 



139 



worm Phascolosoma becomes desiccated by sojourn in sea water 

 more concentrated than that in which it lives, or hydrated by so- 

 journ in diluted sea water. Upon return to normal sea water the 

 body weight in each case reapproaches the control weight (fig. 85). 

 The rates of water exchange gradually diminish (fig. 86), but it may 

 be noted that in positive loads the rates after the first hour are 

 more nearly uniform with time. 



At equal loads the exchanges of water, all of which occur 

 through the body wall, are much more rapid as intake than as out- 



Gephyrean Worm 

 Phascolosoma 



Hours 

 Fig. 85. Course of net water load (% of B„) after return of worms to normal 

 sea water from sea waters of other concentrations. Phascolosoma at 23° C. Each 

 curve represents the average of 2 to 4 tests (5 individuals per test) the points being 

 means of interpolated weights. Additional data of Adolph ('36b). 



put. In relation to water load, each gain appears to be continu- 

 ously proportional to the gradient of osmotic pressure between 

 outside (Pe) and inside (Pi) the body (Adolph, '36b). The gain 

 follows the equation : SW/At = h V"1B (Pj - Pe) , in which the cylin- 

 drical worm has weight B and length 1. The exchanges are at rates 

 roughly proportional to B^''^ or ■\/lB (body surface area), in the 

 range of computed areas from 4 to 20 cm.^ The coefficient h has the 

 dimensions of permeability. 



