380 THE AMERICAN NATURALIST.  [Vou. XXXIII. 
backward (away from the source of stimulus), turns toward the 
aboral side, and swims forward, exactly as in the case of a 
mechanical stimulus. 
2. Stimulus at Posterior End. — The crystal of salt is held 
close to the posterior end. The animal contracts and swims 
backward — therefore toward the source of stimulus. It thus 
either strikes the crystal of NaCl or passes through the 
densest part of the solution; then continues backward some 
distance, finally turning toward the aboral side and swimming 
forward. 
3. Stimulus at the Side. — The animal reacts as in the two 
preceding cases, the direction of motion having no relation to 
the position of the stimulus. 
4. Stimulus Unlocalized.— The Spirostoma are dropped di- 
rectly into 2 per cent NaCl. They contract, swim backward, 
turn about irregularly, and soon die. 
The following experiment, giving results in almost all the 
above categories, is particularly striking. A number of Spi- 
rostoma are placed on a slide in a considerable quantity of water. 
Then a few crystals of NaCl are placed in the center, the 
cover-glass quickly supplied, and the reactions of the animals 
noted. All those in the immediate neighborhood of the NaCl 
soon contract and swim backward, as the flood of salt solu- 
tion diffusing from the crystals comes against them. The 
- Spirostoma are scattered, with axes oriented in no special direc- 
tion, therefore some lie with anterior ends directed toward the 
mass of salt, so that this end first comes in contact with the 
salt solution’; others with posterior end thus directed ; others 
with long axis oblique to the direction of the mass of salt. All 
contract and swim backward, whatever part of the body is first 
met by the diffusing solution of NaCl. Those with anterior 
ends directed toward the mass, swimming backward, of course 
move directly away from the salt. Those with posterior end 
toward the salt, likewise swimming backward, pass directly into 
the densest part of the solution of NaCl, and are quickly plas- 
molyzed and killed. Those with long axis oblique to the direc- 
tion of the NaCl also swim backward, some thus approaching 
more or less obliquely the solution of NaCl; these do not turn, 
