FRESHWATER AND MARINE ORGANISMS. 329 



dilution be carried out with an isotonic solution of a non-electro- 

 lyte. Several species of invertebrates, both freshwater and 

 marine, have been compared in this manner. The flatworm 

 Dendrocceliini and the earthworm Lumbricus, both of which 

 inhabit fresh water, reach the new volume level in about 5 hours. 

 The marine annelid Phascohsonia, when placed in sea water 

 diluted to half its normal concentration, attained the new volume 

 le\el in less than 2 hours. Evidently Phascohsonia adjusts to 

 the new environment more rapidly than the freshwater species; 

 it ha- lc-- re-i-tance to the penetration of water through its 

 -urf.i' 



\\V \\i-h-il to know whether di--ol\cd .-nb-tances likewise 

 penetrated at different rates through the body coverings of these 

 i\\o cl of animals. After the fir-t adjustment to the im- 



nier-ion in -alt solution by loss of \\ater. freshwater anitnal- 

 remained at the ne\v level of volume for -c\cral days, and \\hen 

 replaced in frc-h water recovered two third> ot the original loss. 

 J'/KI^I olnMinia, however, in diluted sea \\aier lost slowly a con- 

 siderable amount of the dissolved substance of the body fluid-. 

 BO that \\hen linally returned to normal sea water very little 

 \\ater pa--d iioin the body compared to that originally gained. 



A -till larger number of animals \ve have compared \\ith 

 respe t to their resistance to solution- ot -alts and other sub- 

 stances. A li-t ot these species, and the hiijiot concentration- 

 of certain -olutc- \\hich they can just -ur\i\e for a considerable 

 length ot time, aie given in Table I. An important conclusion 

 can immediately be drawn from these data; namely, that tor 

 I rc-h \\aier oi^aiii-m- the osmotic pressure of the medium usually 

 limit- -HI \i\al, \\hile for marine organisms a creat ranve ot 

 concentration- i an be resisted. Thus, marine (".ammarn> \\ill 

 li\e indefinitely if transferred to sea water diluted with di-tilled 

 \\ater up to 0.5 per cent. (0.005 M), or concentrated by the 

 addition ot salts up to 160 per cent. (1.50 M, corrected for 

 ioni/ationj. l ; re>h\\ater Gamniarns, on the other hand, are 

 usually killed by imnier-ion in any solution of a concentration 

 equivalent to 0.35 M. Gradual dilution or concentration of the 

 medium does not appreciably extend the-e limit-. 



The general MgnifuMiice of the conclusions from .ill the abo\- 

 experiments we interpret to be that both di>-ol\ed -nb-tanco 



