Water 19 



alternately exposed for several hours twice each day to completely fresh water 

 and then to relatively undiluted sea water. ''■'•' In Plymouth soft tap water 

 Gunda swells rapidly, doubling its \olume in an hour, and if left it dies within 

 48 hours. In dilute sea water or in stream water swelling occurs hut is much 

 less than in tap water. The calcium of the stream water lowers the permeabili- 

 ty of the body surface to water, and this retards swelling. Addition of such 

 substances as sodium chloride, sodium bicarbonate, or glycerol, to distilled or 

 tap water does not retard the swelling. -"' In dilute solutions not only is 

 water taken up but salts are lost. The worms lose 25 per cent of their body 

 salts while imbibing water equivalent to their initial volume. '"■' In fresh 

 water lacking calcium, as in distilled water, salt loss is rapid and general 

 cytolysis occurs. 



When Gunda is in dilute sea water, water passes osmotically through the 

 ectoderm to the parenchyma, which swells during the first hour.-' Then the 

 water collects in vacuoles in endodermal cells lining the gut, and the body 

 volume remains relatively constant. Thus other body cells are kept from 

 becoming diluted and normal activities of the worm can continue. Here, then, 

 is osmotic regulation by storage of water in endodermal vacuoles, but body 

 volume remains high. The vacuoles remain as long as the worm is in a dilute 

 medium. Why they do not continue to grow beyond a certain size is not 

 entirely clear. Beadle cut worms behind the pharynx and found that the 

 ectoderm closed over the wound, thus preventing any opening to the exterior 

 from the gut. Such posterior ends swelled and reached equilibrium size in 

 almost the same manner as normal worms in water containing calcium. Thus 

 the water must normally enter through the ectoderm and cannot leave by way 

 of the gut. Oxygen consumption is greater when the worms have reached 

 equiUbrium in dilute sea water. -^ Also, in anaerobic conditions or in the 

 presence of cyanide, swelling is greater than in normal aerobic conditions. 

 Beadle thinks that the active process which occurs on adjustment at a given 

 volume is due to energy expended in decreasing permeability of the ectoderm, 

 although, as Krogh points out, Beadle has not proved that the excretory organs 

 are not used. Also energy must be expended by the gut cells to keep the 

 vacuoles dilute. In any case, Giinda swells in fresh water; the excess water is 

 taken up by vacuoles in the gut cells, and then an active process, either 

 decreased permeability or increased excretion, takes over to prevent further 

 volume increase, and even allows some decrease in size. 



Osmotic Regulation by Exclusion of Water. The eggs of marine inverte- 

 brates swell and shrink to correspond with changes in the medium. The eggs 

 of marine teleosts are hypotonic to sea water, whereas all fresh-water eggs are 

 hypertonic to their medium. Osmotic relations of embryos have been discussed 

 by Needham,!^*'- ^" and by Krogh.^^-^ 



The egg of the killifish, Fundidiis, is normally more dilute than its marme 

 medium. '''- These eggs are hvpertonic in dilute and hypotonic in concen- 

 trated media, isotonicitv occurring at about Af,>.=0.76°. Such small changes 

 as occur in diflferent tonicities are attributed to adsorption by the outer mem- 

 branes, 152 and permeabilitv to water is very low. Eggs of the plaice show 

 limited permeabilitv to both water and chloride. ''' Krogh found that the 

 extra-embryonic fluid of Nerophis was in equilibrium with sea water {^11 

 mM), whereas the embryo was much more dilute (180 mM). 



