OSMOTIC PRESSURE OF SEA WATER. 263 



in the osmotic pressure, viz., the interchange of water, and of 

 salts. 



The entrance of water is proven by the enormous increase in 

 weight, and the swollen appearance of those animals which have 

 been placed in diluted sea water. This swelling has already been 

 referred to (Table IV.) as noticeable in Astcriasand the two worms 

 Nereis and Chcetopterus. In one experiment in which a Liiniilns 

 was kept for sixteen hours in fresh water the animal became so 

 swollen that the gills burst and the water of the aquarium be- 

 came blue from the haemocyanin of the exuded blood. The 

 blood of the animals subjected to diluted media became notice- 

 ably less viscous and owing to its increased volume and the high 

 internal pressure, was easily obtained from the animal. When 

 subjected to concentrated sea water it was often difficult to secure 

 sufficient blood from the lobster to make the desired determi- 

 nations. That an exchange of salts also takes place, although 

 far less rapidly than the exchange of water, is shown by the fact 

 that when the animals are placed in distilled water, chlorides are 

 eliminated and a copious precipitate is obtained upon the addi- 

 tion of silver nitrate. No quantitative chemical analyses of the 

 aquarium water were made but in one such case an increase in 

 the osmotic pressure was indicated by the change in the freezing 

 point ; Limulns was the animal experimented with and after 

 twelve hours' immersion the freezing point of the aquarium water 

 had been lowered from 0.02 to 0.23. 



Quinton, 18 9 experimenting with Aplysia, has also found an in- 

 crease in weight when the animal is subjected to dilute sea water 

 and a loss in weight in concentrated solutions, and he has further, 

 by chemical analyses, found loss and gains in the amount of salts 

 in the blood of this animal just equalling the respective gain or 

 loss from the aquarium water. Fredericq 3 > 7i 8 ' made similar 

 analyses. 



There are many other proofs of the permeability of the inverte- 

 brate membranes to various salts. Loeb's I5 experiments on the 

 rhythmic contractions of medusae (Gonionemus) indicade the per- 

 meability to NaCl, CaCl 2 , and KC1. The death of invertebrates 

 is easily induced by acids and the salts of the heavy metals. 

 Fredericq 9 has placed potassium ferrocyanide and nitrates in the 



