26 University of California Publications in Zoology [Vol.19 



and remain there. The "records of position" obtained during the 

 descent are entered as those that show movement. But if after the 

 bottom is reached the animal remains there it is good additional 

 evidence that it is positively geotropic. It is jtossible, of course, that 

 an animal which remains at the bottom may be too fatigued to ascend 

 although it is really negatively geotropic. I believe, however, that 

 sufficient precautions were taken in these experiments to guard against 

 such occurrences. The plus and minus signs at the top of the "no 

 movement" columns refer to the direction of the last recorded move- 

 ment. If the animal is in a vertical tube the plus sign in the "no 

 movement ' ' column means that after a descent of greater or less extent 

 there had been no movement upward at the time the next record was 

 put down. Likewise, the minus sign indicates that following a move- 

 ment upward there was no descent. There is nothing in the summary 

 in the tables to show at what level the animals remained stationary; 

 but, as a matter of fact, most records that show no vertical movement 

 are for animals on the bottom of the container or at the top of the 

 column of water. Some records of position that show no movement 

 were obtained because of what appears to be a thigmotactic reaction 

 against the walls of the aquarium ; at such times the animal is sta- 

 tionary. But these records are not numerous enough to change the 

 general results. 



As regards the "per cent of animals moving" (table 4, for ex- 

 ample) it doubtless will be perplexing to the reader to note that the 

 sum of the two percentages under the heading may exceed 100. In 

 explanation it may be said if an animal moved at all toward the nega- 

 tive end that fact was noted, and if there was any movement toward 

 the positive end that also was set down. It does not matter how many 

 times or how far an individual is recorded as moving in one direction 

 or the other ; if the animal changed position once or a hundred times 

 the tally would show 1 in either case, so far as "per cent of animals 

 moving" is concerned. For example, it is possible that one individual 

 in a set of five may be recorded as having changed position once toward 

 the positive end and twenty times toward the negative end ; but that 

 particular specimen would count as one animal in the negative column 

 and one in the positive column. The other four may all have moved 

 toward the negative end only, and in the summary of the behavior 

 of the set the figures would be: positive 20%, negative 100%. This 

 example is not an actual one, but it ought to show why it is that such 

 results appear as those in the fifth and sixth columns of table 4. The 

 same explanation holds for other tables of the form of table 4. 



