C. B. Davenpoet 17 



The inside of the sand spit is a region of sedimentation. Plants grow here, 

 and here the plant-feeding snails and fiddlers live. The organisms that are found on 

 the beach are not accidentally there, nor is the fauna determined by causes remote 

 and too complex to be unraveled. The fauna is determined by definite proximate 

 causes of a simple sort that act, the world over, in the same way, and so give to a simi- 

 lar sea beach in other parts of the world a similar collection of animals — excepting 

 that each species may be replaced by another. 



COMPARISON OF COLD SPRING BEACH WITH THAT OF LAKE MICHIGAN NEAR 



CHICAGO 



The question arises : How far is the fauna of the sea beach determined by the 

 beach conditions of sand, sunlight, and proximity to a body of water with its strand 

 zone of debris? Will beaches in general, whether of a fresh-water lake or of the sea, 

 tend to have the same fauna ? To test this question I have examined the fauna of the 

 shore line of Lake Michigan, south of Jackson Park, Chicago. Here one finds a 

 sandy beach essentially like that at Cold Spring Harbor. On one side extends a huge 

 body of water which differs from that of the harbor chiefly in its lower specific gravity 

 and in the absence of marked tides, but resembles it in its waves. We may recognize 

 here a submerged beach and a terrestrial beach. 



I. FAUNA OF THE SUBMERGED ZONE 



This inundated part of the beach supports, as one would expect, a fauna the 

 species of which are quite unlike those of the sea. Yet we may recognize a sessile 

 fauna, a crawling fauna, and a swimming fauna. 



a. The sessile fauna includes here, as in the sea, the lamellibranchs. These 

 belong to two families, the Unionidae and the Spheridae. These, like their marine 

 allies, feed on minute organic particles, chiefly algae. The Unionidae are the large 

 forms and seem to take the place of the marine Mactra, Venus, and Mya. They 

 occur in the streams and lakes of all parts of the northern hemisphere, but the group 

 is best developed in North America. The Spheridae are small, and take the place of 

 the Nuculas of Cold Spring Harbor and Donax of our southern seashore. They are 

 found in the streams and lakes of all countries. 



b. Crawling animals belong chiefly to the groups of gastropod Mollusca and 

 Crustacea — the Echinoderma being wholly absent. The snails are mostly small and 

 seem to replace the Littorinas and the Rissoas of the seacoast. The principal crawl- 

 ing crustacean is the isopod Asellus communis, which lives on the wood and among 

 the roots of the shore line. The group is, indeed, poorly represented here as com- 

 pared with the sea. Burrowing animals seem to be almost entirely absent, possibly 

 on account of the absence of such predaceous forms as occur in the sea. 



c. The swimming animals are here, as in the sea, partly scavengers and partly 

 predaceous. First of all we have in the water above the shore numerous Entomos- 



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