3OO VICTOR E. SHELFORD. 



Anodontoides (Fig. 15). Moves in the direction headed. 

 Sphcerium (Fig. 16). Moves in the direction headed. 



3. METHOD OF READING. 



At the time of each reading the number of animals headed 

 up stream within approximately 16 degrees of the direction of 

 the current was counted positive. Those headed down stream 

 were counted negative; those not falling within an arc of ap- 

 proximately 32 degrees of the total possible orientations for both 

 positive and negative were counted as indefinite. Mollusks with- 

 drawn within their shells and other animals lodged against the 

 screen were counted as inactive or out of the experiment. Read- 

 ings were taken fifteen seconds apart; fifteen seconds being suf- 

 ficient time for individuals of all the species to orient at least 

 once, most of them several times. In nearly all cases twenty 

 readings were taken. With the exception of the snails, ten 

 readings were taken, then the animals were disturbed, loosened 

 from their footing, and after a short period they were read ten 

 times again. 



4. PROGRESS UP STREAM. 



The preliminary experiments, especially those of Mr. Finley, 

 showed that all the species make definite progress up stream. 

 This results from (a) positive orientation and (b) movement. The 

 amount of movement up stream differs for different species and 

 for the same species under different conditions. Thus positive 

 orientation, the first essential to up stream movement is the more 

 significant. 



5. TYPICAL RESULTS. 



The diagrams illustrate typical orientation results, in per cent, 

 of total. The upper diagrams (Fig. 18, 19, 22, 23, 26, 27) give 

 per cent, positive, negative, indifferent and inactive (or on the 

 screen, shown blank). Since inactive individuals and individuals 

 on the screen cannot be regarded as responding to the current, 

 the lower diagrams (Figs. 20, 21, 24, 25, 28, 29) give per cent, of 

 active individuals showing positive, negative, and indefinite 

 orientation. The data on the active individuals were used as a 

 basis for comparison. Since Anodontoides and Sphcsrium move 



