156 Papers from the Department of Marine Biology. 
SUMMARY. 
A pigment, ‘‘antedonin,” occurs in the epithelium of the viscera of 
the ‘prickly fish,”’ a holothurian, Stichopus ananas, which may be used 
as an indicator for the penetration of acids. 
Living tissues are resistant to the penetration of all acids except 
salicylic, benzoic, and possibly valeric. The degree of resistance 
varies with the acid and is not nearly so marked as in the alkalies, 
which are compared with the acids. 
The penetration rate of HCl is roughly proportional to the concen- 
tration. Dead tissues are readily permeable for all acids. 
There is no relation between the degree of dissociation of the acid 
and its rate of penetration or between degree of dissociation and tox- 
icity. 
There is a general relation between penetrability and lipoid solu- 
bility and capillary activity, but it is not exact and not quantitative. 
The best relation is between penetrability and toxicity. The acids 
which penetrate most readily are most toxic, irrespective of their 
strength. 
With acids as with alkalies lipoid solubility or capillary activity 
appears to be a determining factor in penetrability. No distinction 
can be drawn between these two possibilities, since the penetration 
series is not in exact agreement with either lipoid solubility or surface- 
tension series. Acetic, propionic, and butyric acid are most out of 
place and it is suggested how this might be explained if the indicator 
were dissolved in a fat solvent or the cell as a whole acted as a fat 
solvent. 
If an acid is soluble enough in fatty substances, it encounters no 
resistance at the cell-surface; if it is lipoid-insoluble or only slightly 
soluble the normal cell-surface must be destroyed before it can enter. 
The strength of the acid and possibly also a specific action of the 
anion on the surface protein will then determine its rate of entrance, 
or, better, its rate of alteration of the cell-surface. 
Cells behave toward acids and alkalies as if they were droplets of a 
fat or fat solvent, which suggests that they must be composed largely 
of fat-protein combinations in which the visible physical character- 
istics of fat are masked. 
