BIOLOGY OF PHYSA 29 



creek in which Elodea grow in less abundance contained the 

 snail, and the inference is suggested that the presence of this plant 

 in abundance is not favorable in its food or oxygen relation to Physa. 



Char a, Ceratophyllum, Hypnum, and Elodea are the more 

 common pond weeds met in this study and it is a notable fact 

 that they occupy the water of their optimum habitat almost to 

 the exclusion of any other plants although many ponds of a few 

 feet in area were found where they were all growing together. 



The scarcity and often the entire absence of Physa is also 

 noted where Ceratophyllum, Chara and Hypnum are growing in 

 abundance. An abundance of these plants growing in a small 

 aquarium will cause Physa to die within a week or two. Yet 

 a moderate amount of these plants in an aquarium will support 

 the snail in numbers. It therefore seems probable that it is 

 the abundance of the plant and not the kind of plant that proves 

 so disastrous to Physa. It also seems probable from the obser- 

 vations made in both field and laboratory, that relatively few 

 plants in quiet water will serve both for food and aeration and 

 that where the optimum pond weed habitat occurs Physa is 

 near its minimum, and that the optimum of the snail is not 

 the optimum of the water weeds. It may be remembered that 

 the only places found where the plant optimum and the snail 

 optimum closely approached each other were those in which 

 water coursed gently over the weeds as in the brook ditch shown 

 in Fig. 2, and those in which six to eight inches of water stood 

 above the weeds, as at Geddes, — Fig. 7. In these cases, it 

 would seem that whatever elements the plant growth added to 

 or subtracted from the water was neutralized by the inflowing 

 fresh water. 



The following general conclusions seem probable from the 

 above observations concerning the favorable conditions for 

 plant growth, and their general relation to the snail habitat: 



1. Where the pond weeds have captured quiet waters, there 



were found no snails living or dead. 



2. Snails live in moderate numbers where there is a luxuriant 



growth of weeds if there be a considerable depth of 

 water above the plants, or if water is gently flowing over 

 them. 



3. The snails occur in the greatest numbers where there is a 



moderate amount of water plants and organic debris. 



