C. B. Davenport 9 



stones (Fig. 4). It is also a region where oxygenation is combined with abundant 

 moisture, so that conditions peculiarly favorable for respiration would seem to be 

 afforded. 



A fine layer of silt is dropped over the whole surface with each flood tide, afford- 

 ing thus abundant but microscopic food. But, on the other hand, it is a region of 

 great exposure to terrestrial animals ; so that only the stratum a little below the surface 

 offers great safety. Also the lower beach is a region of wave action which makes it 

 difficult for animals to secure a permanent place on it. Finally, and most important 

 of all, the waves and currents cause ti^ sands to shift, and this adds to the difficulty 

 of maintaining a foothold. Conj^Mently there are but few animals living on the 

 lower beach, and such as there. ^jf^^ive a curious and very strenuous life. 



All over the lower beach will be found, upon careful examination, large num- 

 bers of extremely minute and active insects belonging to the group of Thysanura. 

 These are arctic forms of Collembola of the species Xenylla humicola, O. Fabr. and 

 Isotoma besselsii, Pack., together with an occasional Anurida maritima, Guer. 1 These 

 Collembola are feeding on the rich microscopic debris which has been dropped on the 

 lower beach. Being insects, they are air breathers; and the question arises: What 

 do they do when the tide comes in? To answer this question I made measurements 

 of the area occupied by the Collembola at different stages of tide. At tides lower than 

 one-half tide the upper limit of the Collombolan zone is about nineteen meters north 

 of the storm-cut bluff, or about ten meters north (i. e., seaward) of the mean high-tide 

 line. The lower limit is about two meters from the momentary tide-line. As the 

 tide retreats the lower limit follows, while the upper limit remains constant. Thus, 

 on September 10, 1901, with falling tide, the following determinations were made: 



Note that the lower limit of the Collembola travels down pari passu with the 

 tide. 



As the tide rises, the Collembola tend to retreat before the edge of the water, so 

 that they are even crowded together there. Thus, on September 10, at 7 A. M. (one- 

 third tide, rising), the lower edge of the Collembolan zone was about three meters 

 away from the water's edge. As the tide rises still higher they crawl into the sand, 

 until, at high tide, most of the Collembola are under the sea. But not all of the Col- 

 lembola are there. At high tide one finds some of them floating on the quiet water out 



1 For the determination of these species I am indebted to my friend, Dr. J. W. Folsom, of the University of Illinois. 

 (Cf. Wahlgren, 1899; Folsom, 1901.) 



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