BIOLOGY OF PHYSA 7 



has a general but not a uniform occurrence in the creeks exam- 

 ined. To this statement the objection may be raised that 

 the snail may be found in these habitats at one season but not 

 at another, or during one year and not another, so that a de- 

 scription of the present habitats leads to nothing. Moreover 

 it may be said that natural and artificial changes are going 

 on continually and the snail is bound to become dispersed or 

 destroyed, as the old habitats disappear and new ones arise 

 through these modifying agencies. However, the question 

 which I wish to discuss is primarily whether there is a combin- 

 ation of like physical elements which are met with in every 

 flourishing normal habitat and if so in what way these elements 

 are changed by natural causation. 



a. Creek Bed Habitat. For convenience, I shall describe the 

 optimum creek habitat first, and follow with those of less im- 

 portance. 



The small creek in question lies northeast of Ann Arbor 

 along the Toledo & Ann Arbor R. R. track, on the edge of the 

 city. It rises a few miles north of the city and flows into 

 the Huron river near the City Park. This creek meanders 

 for a distance through its flood plain, cutting from the con- 

 cave and building up on the convex side. At times it carries 

 a load of silt or deposits it in some quiet pool, and again sorts 

 gravel and stones and leaves them in heaps in its bed. In Fig. 3, 

 is shown a load of stone deposited and it is here that the optimum 

 Physa habitat occurs in this stream. The bank on the left 

 side of the picture is very gently sloping while that on the right 

 side is somewhat steep. The stones cover the bottom at this 

 point and the water flows quite rapidly over them. No large 

 water plants grow above or below this immediate spot, but 

 Cladophora grows in abundance upon the submerged stones, 

 in felt 'like masses. The Cladophora shows a well marked cor- 

 relation in size and abundance to the velocity of the stream. 

 Wherever the current strikes with full force upon the stones, 

 the plants are short, but on the sheltered sides they grow more 

 heavy and abundant. The stones are placed in such relation 

 to each other that one or more of the faces of each is protected. 

 Many of them are so placed that they also protect the bottom 

 of the creek which lies between them. The current strikes 

 the first stone squarely and breaks over its top and in turn 



