Water j 3 



at least 24 hours, although there may be a slight volume decrease at 48 hours 

 (Fig. 4). Chloride concentration in the animal fell by 16 per cent in the same 

 time. ■'' Another gastropod, Onchidhtin, shows similar swelling. " In a 

 mixture of one-third sea water and two-thirds isotonic sucrose the volume fell 

 by 33 per cent in 14 hours; this shows that salt can be lost. However, Onchidi- 

 um regained its original weight when returned to sea water from dilute sea 

 water, hence salt loss must have been negligible. Doris and Onchidhini show 

 practically no volume regulation within a period of a day. 



MytUiis ednlis is a bivalve which can live in a wide salinity range. The 

 hemolymph of Mytiliis edidis adjusts-^-' to the medium as follows: 



Ao Ai 



0.94 0.95 



0.77 0.77 



037 0.35 



Osmotic adjustment occurred in Mytihis in sea water diluted by 25 to 41 per 

 cent, although the hemolymph did not become diluted proportionately when 

 the sea water concentration was reduced by 70 per cent. ''^ If the valves are 

 open, Mytdiis gains weight in a hypotonic medium and shows no tendency to 

 return to its original volume during 50 hours (Fig. 4). '"^ However, if the 

 valves are closed the weight is unchanged; "''^ in Vemts with valves closed no 

 dilution of the blood occurs during many days in h\potonic sea water. ^^ The 

 chloride concentration decreases in Mytihis in dilute sea water and increases 

 in concentrated sea water "^ along a curve similar to that of the volume 

 change. ^''^ In 52 per cent sea water weight increased by 25 per cent; at the 

 same time the total tissue chloride content decreased by 44 per cent and the 

 total water content increased by 18 per cent. "^ Krogh ^^^ reported similar 

 changes in water and chloride content in 76 per cent sea water, but in 30 per 

 cent sea water he reported an increase of about 32 per cent in water content 

 and a decrease of 71 per cent in chloride content of hemolymph. Unfortunate- 

 ly, data on weight, osmotic concentration, and chloride- are not available for 

 the same individuals, but evidently Mytilus adjusts to its medium osmotically, 

 part of the change being by water uptake or loss and a larger part by chloride 

 loss or uptake, the chloride loss or gain occurring at approximately the same 

 rate as the water exchange, body volume being incompletely regulated. 



The holothurian, Caiidina, swells in dilute sea water and shrinks when 

 transferred from dilute to normal sea water. ^■'•' The \olume change in each 

 transfer is less than half the value predicted if the body wall were semiperme- 

 able. Actually chloride is transferred in and out, but the rate of water ex- 

 change is greater than that of the gain or loss of chloride, and volume changes 

 persist. 



Osmotic Adjustment with Volume Regulation. Many marine invertebrates 

 adjust osmotically to the concentration of the medium, but, in addition to gain 

 or loss of water, they also gain or lose salts so that the body volume is kept 

 relatively constant. 



The blood concentration of those marine molluscs which have been exam- 

 ined is equal osmotically to the concentration of their medium. Bethe"*-*- ^^- -^^ 

 studied weight and ionic changes of the gastropod, Aplysia. On transfer to 

 dilute sea water the weight rises at first (Fig. 4), as water is taken up, and 



