BIOLOGY OF PHYSA 41 



that may be used as food and in death they yield important 

 salts and organic substances to the water." 



However this may be for plankton, few or no snails are found 

 where there is an abundance of both rooted and non-rooted 

 gross vegetation. Nor can their scarcity in these localities be 

 attributed to the lack of food since they eat the plants and prob- 

 ably do not take nourishment from solution in the surrounding 

 medium. It is therefore evident that some reason other than 

 the one given by Kofoid and Pond for plankton, must be sought 

 to account for the inimical effect of excessive vegetation of the 

 submerged sort on snails. 



The effect of drought on the water plant is of great importance 

 to the snail. To just what extent drought kills water weeds 

 is an unsolved problem so far as is known to the writer. That 

 drying kills the plant itself, there is no doubt, since it is easily 

 demonstrated experimentally, but there is no experimental 

 evidence known to me of the extent to which spores and seeds of 

 these plants can resist drought. To be sure Ceratophyllmn and 

 Elodea produce seeds rarely and when they do so, it is in the 

 middle or latter part of the summer. It is in this part of the 

 season, however, that drought usually occurs in the ponds. 

 Taking all together, the chances indicate that drought is fatal 

 (at any rate in this region) to such pond weeds as Elodea, 

 Ceratophyllum and Chara, since all the ponds known to dry up 

 are devoid of these plants. 



Means of \\ithstanding drought are sooner or later of great 

 importance to snails that inhabit ponds or swamjDs. It is seen 

 that a pond or swamp may dry up, but as long as the ground 

 remains damp, all the above mentioned snails may live. All 

 species of mollusks studied, except the Uttle Sphaeriuni, ean 

 live if there is a clay substratum for some weeks even in severe 

 drought. The pond near the University campus is a good 

 illustration of this. 



Where the substratum of a pond or swamp is of loose texture 

 and there is but a small amount of clay present, few snails are 

 likely to survive and restock the pond because the sealing is 

 a matter of chance. The formation of an epiphragm greatly 

 enhances the chance of survival in time of drought irrespective 

 of the substratum. A pond with a sandy substratum was ex- 

 amined and no snails but Lymnaea were found and no pond 



