MICROSCOPIC FAUXA OF SAXDY BEACHES 



99 



distribution of tardigrades in three dift'er- 

 ent series of sand samples which indicate 

 to some extent the diversity of conditions 

 found. In some instances maxinnim num- 

 bers were found 50 cm from the edge of the 

 water; at other times, even in the same 

 beach, maximum innnbers were found 200 

 cm from the lake regardless of the distri- 

 bution of food in the sand. From the data 

 gathered, however, we conclude : 



1. Of all Metazoa, the tardigrades ap- 

 peared to be most susceptible to drying con- 

 ditions and consequently were the most re- 

 stricted horizontally. Nevertheless, there 

 was no direct correlation between their num- 

 bers and the amount of water in the sand. 



2. Only small numbers of tardigrades 

 were found immediately at the edge of the 

 water and very few were in the submerged 

 sand within 50 cm of the edce of the water. 



;i None were found in the beaches of 

 Lake Michigan and Lake Superior because 

 of the very low food supply in those sands. 



4. On the other liand, it is significant 

 that the larger populations were found in 

 beaches where there was less than 4.0 mg of 

 particulate organic material per 10 cc of 

 sand. 



5. As indicated in Table II, the largest 

 numbers were found in the top centimeter of 

 sand, while 78 per cent were present in the 

 top 3 cm ; one per cent were found at a 

 depth of 8 cm and only a few stragglers 

 deeper than that. 



6. The beaches of soft-water lakes con- 

 tained greater numbers than those of the 

 hard-water lakes. Whether this is due to 

 the chemistry of the interstitial water, to 

 food relationships, or to some other factors, 

 is not known. 



TABLE I 



DlSTRIBUTIOX OF TARDIGRADA IX THREE SERIES OF SaXD SAMPLES. EE.SULTS ARE EXPRESSED AS XUilBERS 



OF ORGAXisiis Per 10 cc Saxd Sample. 



