84 The Animal Mind 



seem to be excited to their feeding reaction by a combination 

 of mechanical and chemical stimulation. They are very 

 sensitive to slight water disturbances, and react by stopping 

 the respiratory movements if a needle is touched to the 

 surface of the water above them. Food juice diffused 

 through the water makes them very active ; while in this 

 state they will attach themselves to a glass rod, but drop 

 off at once. When they attach themselves to food substance, 

 however, they hold on with traditional tenacity. Chemi- 

 cals of various kinds produce withdrawing reactions, and 

 Lohner (438) finds evidence that leeches experience " taste 

 compensation." When we have to eat sour fruit we can- 

 cel the sour sensation by putting sugar on the fruit. A 

 five per cent, sugar solution produced withdrawing reac- 

 tions in a leech, but if the sugar solution was mixed with 

 a nine per cent, salt solution, its strength had to be raised to 

 seven and five tenths per cent, before the leech reacted to it. 



19. The Chemical Sense in Mollusks 



In the case of the Mollusca there is little satisfactory 

 evidence on the subject of the chemical sense. The 

 Acephala, to which the clam, oyster, and scallop be- 

 long, do not take food by active movements; hence, 

 of course, they can have no specific feeding reactions. 

 Chemical sensibility, distributed over the surface of the 

 body, has been observed in lamellibranchs, a branch of 

 the Acephala (522). Gasteropods, including snails and 

 slugs, have, owing to their active food taking, more use for 

 a chemical sense ; in marine snails it seems rather definitely 

 localized in the feelers (522). Yung found in the snail 

 Helix pomatia that smell was most acute at the end of the 

 feelers, but that the animal even when deprived of its 



