342 S. O. MAST 



pools, and ordinarily does remain at the water's edge is rarely 

 if ever caught in them. 2 It was this question that inspired the 

 following experimental observations, all of which were made 

 during the summer of 1914 on a sand beach at Beaufort, N. C. 



I shall first give in a general way the results of these obser- 

 vations, all of which were repeated a considerable number of 

 times; then I shall present a few experiments in some detail. 



If Fundulus gets into a tide-pool while the tide is rising it 

 usually swims about in a deliberate sort of way, stopping here 

 and there to root in the sand and to play with its companions. 

 This behavior continues until the tide turns or at any rate 

 until it is very nearly high. After that the animals may still 

 swim about much as they did before, but they invariably, every 

 few moments, return to the outlet of the pool and swim out and 

 in again. Thus they continue to test the depth of the water 

 in the outlet, and as soon as it gets too shallow they leave the 

 pool and do not return. This accounts for the fact that they 

 are not caught in these pools under ordinary circumstances. 



But what interested me primarily was the behavior observed 

 in pools in which the outlet had been closed before the fishes 

 had escaped. Under such conditions it was found that the 

 behavior depends very largely upon whether the water is running 

 in or out of the pool at the time it is closed. 



If the water is running in, nothing phenomenal occurs. The 

 animals may swim about rather rapidly for a few moments, 

 but even if they do, they very soon become quiet and proceed 

 to feed and play in their accustomed manner. This experiment 

 was repeated many times and only in one case was the behavior 

 essentially different from that described, and in this case the 

 tide was very nearly high at the time the pool was closed. The 

 response observed under these conditions lead to some important 

 conclusions that will be stated later in connection with a detailed 

 description of the experiment. 



If the water is running out when the pool is closed the be- 

 havior of the fish is quite different from what it is if the water 

 is running in. After the pool is closed under these conditions, 

 they first swim about rapidly in various directions for a few 



2 1 have again and again, during the ebbing tide, examined numerous tide-pools, 

 but I have never found Fundulus in any of them after the water had stopped flow- 

 ing out although in some instances they were still 40 to 50 meters long and con- 

 tained water 20 to 30 cm. deep. 



