14 JEAN DAWSON 



water, but this alone does not seem to account for their scarcity 

 in shallow protected water such as is found in the overflow 

 at Geddes and the bayous above Ypsilanti. There is no 

 perceptible current in either of these places and still there 

 are found comparatively few water weeds and very few snails. 

 In the bayous and in the backwater or "overflow" where there 

 is direct communication with the river waters, there is a con- 

 siderable deposit of silt which may probably account for their 

 absence. It 'is only in the waters that are separated from the 

 river that Physa flourishes. It is evident that as the bayou 

 is cut off from the river by the deposition of material across 

 its mouth that its waters will be less and less influenced by 

 the inflow and silt from the river. As the bayou becomes sepa- 

 rated from the river, it will take on more and more of the char- 

 acters of an ox-bow pond (conditions similar to those of Fig. 6) 

 and like it will furnish favorable conditions for Physa. When, 

 however, complete isolation takes place, then the conditions are 

 essentially like those of a pond. 



Enemies. Cases have been cited above where fish and ducks 

 were thought to prey upon Physa. It would seem that an animal 

 that is as fertile as this snail, would in a comparatively short 

 time, be very abundant in all available places in the creeks 

 and rivers unless destroyed by enemies. The following ob- 

 servations upon the enemies of snails bear upon this point: 

 Baker (1895) says: " Ducks, geese, swans and many other 

 kinds of wild fowl are very fond of snails, and I have never 

 failed to find the remains of them in their crops." Again he says: 

 " Various species of fish feed upon snails as a regular diet. After 

 feeding in one spot they repair in schools to some particular 

 spot often many miles distant from the feeding ground 

 where they digest the animals and eject the shells through 

 the mouth or faeces." Tryon (1882) remarks: "The frog 

 and water lizard eat the fresh water snails." . . . . " The species 

 of Perthostoma, an American aquatic hemipterous insect, eat 

 large quantities of Lymnaea, Physa and Planorbts, which 

 they hold with the fore-legs b}^ folding between the thighs and 

 tibiae; even the larva of this insect, shortly after escaping from 

 the egg, will seize and devour one of these mollusks with as 

 much ease as if schooled in the process a long time." Needham 

 (i 901 , p. 405) records the following for Little Clear Creek : " Snails 



