288 SELIG HECHT 
_Closer measurements as exemplified by Exp. VIII.5 showed that 
“seawater concentrated to be isotonic with 1.57M solution of 
NaCl just stimulated. 
The species is also not very sensitive to diluted seawater. A 
mixture of seawater and fresh water in equal proportions did not 
result in a reaction. The animals of Exp. VIII.5 gave the first 
response when stimulated with a mixture of 3 cc. of seawater 
and 7 ce. of fresh water. This 30 per cent seawater corresponds 
to a 0.19 M solution of NaCl. 
The range over which Ascidia is insensitive to changes in 
osmotic pressure is really not as large as it would seem from these 
experiments, because the concentration of the stimulating solu- 
tion changes before it reaches the receptors. 
To gain some information on the extent of this change, I de- 
termined the toxicity of various dilutions of seawater to Ascidia. 
Eighteen animals of nearly~the same size were placed in lots of 
three in individual bowls containing 300 cc. of the following 
dilutions: 100 per cent, 90 per cent, 80 per cent, 70 per cent, 
60 per cent and 50 per cent seawater. The solutions were freshly 
replaced three or four times daily. 
In the 100 per cent seawater one individual died in less than 24 
hours; the other two were still vigorous after 72 hours, when the 
experiment was discontinued. Exactly the same occurred in the 
90 per cent seawater. The average longevity of the three in- 
dividuals for each of the other dilutions are given in table 4. 
From these results it is clear that 80 per cent seawater is de- 
cidedly toxic to the animals. It is generally true that an animal 
will be stimulated by a solution which is obviously toxic for it. 
TABLE 4 
Longevity of Ascidia in seawater of different dilutions 
CONCENTRATION PER CENT SEAWATER LONGEVITY 
minutes 
100 co 
90 co 
80 960 
70 600 
