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 HC1 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. 



