1SS E. II. HARPER 



Paramoecia is measured by comparing their behavior with an 

 ordinary control. Control experiments in ordinary water may 

 make quite complete surface aggregations after a considerable 

 time, but usually the reaction is gradual, while the treated 

 cultures move up often with surprising quickness. I have had. 

 cases of nearly complete surface collections of treated animals 

 with only a few scattering individuals not included, but when 

 these occurred after a half hour or more I was inclined to suspect 

 the denseness of the aggregation was due to the formation of 

 an acid area about them. Distilled water also has its disad- 

 vantages in causing their ultimate death and some amount of 

 contraction of the body and sluggishness of movements. Accord- 

 ing to Jennings the avoiding reaction given when starting to 

 move downward is the geotropic response per se. It would tend 

 to produce an effect of " orientation by exclusion " to use his 

 terminology. It appears to the writer that the mode of passive 

 orientation would only supplement the geotropic response and 

 not exclude it. The animal might swing passively into the 

 oriented position during its ordinary movements and since this 

 would be a position of stable equilibrium, it might happen under 

 certain conditions, — say in pure water, that movement out of 

 the position of stability might be able to induce a stimulus. 

 Orientation by exclusion is not permanent. But a position of 

 stability in the water does give permanence to the orientation 

 and accounts for the quite regular movements in parallel lines, 

 which is best shown under the magnetic control by heavily 

 loaded animals in the upward stream. 



Jennings also points out that resting animals on the bottom 

 or in contact with the sides of a vessel or in the surface film all 

 show characteristically the oriented position. This may surely 

 be taken as corroborative evidence for the acceptance of the 

 view that there is a position of equilibrium to gravity which is 

 passively assumed. This fact accords well also with the other 

 observation that the oriented position is assumed by animals 

 moving quietly in narrow spirals and that any disturbance of 

 their movements by stimulation annuls the passive orienting 

 tendency. It must be remembered that the normal reaction is 

 a very delicate one. As Jennings says: "Whatever the cause 

 for the reaction to gravity, the stimulation it induces is evi- 



