BIOLOGY OF PHYSA 21 



of snails decreased quite abruptly at the open end of the spit 

 to the right of the optimum (Fig. 9), and also on the side toward 

 the deeper water, but at a more gradual rate at the closed end 

 of the spit. This gradual decrease of snails is accompanied 

 by an increase of water plants and decaying debris which at 

 this point have lodged in the quiet and undisturbed waters 

 behind the spit. The amount of this debris has been greatly 

 increased by the fallen leaves of willows and button bushes 

 which border this part of the water. 



The distribution of snails near the open end of the bar where 

 protection from the current and well aerated water are at once 

 secured, accords with the distribution of snails behind the ice- 

 rampart formed at Kavanaugh Lake, as above described. The 

 bar habitat has but one gap while the ice-rampart has several 

 corresponding to the one of the spit through which the water 

 from the lake has free access. 



Analysis of the elements making up the optimum spit and 

 the bar habitat: Gently sloping shore; an unusual amount 

 of shallow water from the lake; protection from the severe 

 sweep of the weaves ; comparatively few water weeds ; unshaded ; 

 very little decaying debris ; and no fish or other enemies so far as 

 observed. 



The conditions elsewhere in this lake are as follows : In waters 

 wholly cut off by bars from the lake and where many water 

 plants and deciduous leaves make the waters foul, there is little 

 or no snail life found. Thus in one spit-formed pool the water 

 is so laden with deciduous leaves that they had buried all the 

 water plants except a small amount of Hypnuni. This spit is 

 curved and was doubtless formed by alternating currents which 

 turn its point landward, making a hook. It has still a small 

 opening to the south and before the bushes invaded the bar 

 and shore it apparently furnished a favorable Physa habitat 

 but as it stands today no snails are to be found in its foul waters. 

 Another example of this sort is found on the south side of the 

 lake where several approximately parallel bars have formed, one 

 beyond the other, and where but little fresh water from the 

 lake has access. The same conditions prevail that are found in 

 all the waters that are cut off and have become filled with vege- 

 tation and decaying debris ; they are devoid of snails. A moder- 

 ate number of Physa are found in some of the small bays that 



