492 LECTURE XXI. 



impermeable to chlorine ions, and in this way the formation of hydrochloric 

 acid in the stomach was explained. 1 It was soon apparent, however, that 

 such a theory was untenable. If we stop considering the production of 

 the acid by itself, but remember that the formation of the remaining 

 products of secretion point to a specific and extensive activity on the 

 part of the gland-cells, then evidently the formation of the free acid repre- 

 sents only one link in the chain of the entire secretion process. It is not 

 any more remarkable than, for example, the formation of mucin in the 

 cells of the salivary glands. Just as little as we are unable to account for 

 the formation of the latter as a result of purely physical or chemical 

 processes, is it possible for us to understand clearly the production of acid 

 on the part of the cells. 



The question next arises concerning the biological significance of the 

 acid secretion in snails. Troschel, the discoverer of the presence of the 

 acid, was inclined to believe that it was a means of protection against 

 enemies. This is hardly correct in the sense meant. Although the 

 Dolium galea is able to throw out this secretion when on land, the irritating 

 effect of the acid would be lost when the animal is in the water, and, 

 moreover, the water itself offers so much resistance that obviously the 

 animal would not be able to send out a stream of secretion at any desired 

 moment. On the other hand, it is possible that the acid formation, espe- 

 cially on the part of the glands in the skin of the Pleurobranchi, may serve 

 indirectly as a means of protection in the sense that by reason of it these 

 snails will be avoided as food by other animals. Unquestionably, the 

 secretion of the large glands in the gullet must have some other significance. 

 It has been suggested that they play a role in digestion. Careful investi- 

 gation, however, has proved that this is not the case. Semon 2 believed 

 that the sulphuric acid acts upon the lime skeletons of other animals upon 

 which these snails feed. Thus calcium sulphate would be produced. 

 The effect of this decomposition Semon studied on the skeleton of the 

 star-fish. He found that the latter, after it had lain for some time in 

 water containing sulphuric acid, could be broken up by means of the 

 fingers. The direct examination of the intestinal contents of these snails 

 did not lend any support, however, to the assumption of any such action. 

 It is, therefore, improbable that the sulphuric acid is produced for the 

 purpose suggested by Semon. Now we find in the animal kingdom 

 repeated examples of contrivances by means of which one animal which 

 eats another species for its nourishment is able to cripple its prey. This 

 is the case with the poison glands of snakes. Perhaps in the case of the 

 snails the acid forms a weapon of attack. 3 Many sea animals are extremely 



1 Cf. Koppe, Pfliiger's Arch. 62, 567 (1892),andLandislaus v. Rohrer: ibid. 110,416 (1905). 



2 Biol. Zentr. 9, 80 (1890). 



8 W. Preyer: Naturwissensch. Wochschr. Berlin 6, 481 (1890). 



