MOVEMENTS, KTC, OP FRESH-WATIOR PLANA RTANS. 653 



To solutions below this concentration the organisms are 

 either indifferent or, in a few cases, weakly positive. 



Sodium Carbonate, -i- per cent. — Rather weak negative 

 reaction, 



y\) per cent. — Majority of all reactions positive. Remainder 

 weakly negative. 



Ya per cent. — Well-marked positive reaction in all cases. 

 The specimens can be led around the dish by moving tlio tube 

 slowly. 



Below this concentration the reactions were either in- 

 different or weakly positive. 



Summary. — A caustic alkali (NaOH) and a salt of strong 

 alkaline reaction (NaoCOg) produce essentially the same 

 results as the acids. In strong solutions they cause negative 

 reactions ; in weak, positive. 



Salts of Heavy Metals. 



Copper Sulphate, -^jj per cent. — At the very first 

 trials the animals all turned sharply and immediately towards 

 the stimulus (positive reaction), but the solution was so 

 strong as to throw the animal into convulsions, when the 

 head came very near the mouth of the tube. Subsequent 

 trials produced the negative response, 



^ and -jjy per cent. — All specimens give positive reaction. 

 The head is brought up to the tube, and the worm glides up 

 over the latter. 



With all concentrations of CuSO^ there is a very well- 

 marked local' contraction of that part of the bod}^ which is 

 stimulated. 



In another series of experiments with this same substance, 

 the strongest concentration which would produce the positive 

 reaction in all cases was ^^ per cent. 



Zinc Sulphate, -g- and jjy per cent. — Sharp and imme- 

 diate negative reaction, 



■jLy per cent. — Negative reaction, but less pronounced than 

 in former cases. 



-^-^y and -Jy per cent. — Specimens stop when stimulated, 



VOL. 40, PART 4. NKW SElllES. U U 



