12 



Comparative Ayiimal Physiology 



salt movement. Under ordinary conditions, then. Phascolosoma behaves like 

 an osmometer and shows no volume regulation. It is probable that Sipimculns 

 behaves similarly. ■^•' 



50X sw 



^-^VAV 



36 "4 8 



HOURS 



Fig. 4. Percentage change in volume of four molluscs when transferred at zero hours to 

 the dilutions of sea water indicated. Ouchidium changed from 10 per cent to 50 per cent 

 sea water and Aplysia from 75 per cent to 100 per cent sea water at time indicated by 

 arrows. Two patterns of response of Aplysia are shown. Ouchidium data from Dakin and 

 Edmonds," Doris and Aplysia data from Bcthe,''" Mytilus from Maloeuf.'"" 



The polychaetes, Arenicola marina, Nereis pelaoica, and Nereis cidtrifera, 

 show osmotic lability and some swelling in dilute media. The following data 

 -^•* (also Fig. 6) show that Arenicola adjusts in osmoconcentration to its 

 medium both in its natural habitat and in the laboratory: 



Ao 

 0.29 



0.75 

 1.72 



Ai 



0.28-0.30 

 0.76-0.77 

 1.70 



Habitat 

 experimental 

 Kiel Canal 

 Helgoland 



Thus Arenicola must be able to sur\ive considerable dilution of its tissues. 

 The sodium chloride content of the blood of both Sipuncitlus and Arenicola 

 is nearly the same as that of sea water, and since both organisms are isotonic 

 with the ocean the organic solutes must be \ery dilute. "- 



Several molluscs also show little or no volume regulation (Fig. 4). The 

 nudibranch Doris swells rapidly in a dilute medium, and remains swollen for 



