Leeches 43 



in the mud until aroused to activity by the rising temperature of 

 spring. In the vicinity of Philadelphia the great majority of them 

 may be found during the winter occupying a zone between the 

 depths of eighteen and thirty inches. Some go beyond these depths, 

 but few or none into deep water. The advantages of this distribu- 

 tion is obvious. As the heavier cold water first accumulates and 

 longest remains in the depths, animals of the physiological type of the 

 leech must enjoy a longer season of activity because their winter 

 quarters lie in the shallows. Because the ponds probably freeze to 

 a somewhat greater depth in the New York Highlands than at 

 Philadelphia we may expect that the leeches will occupy a cor- 

 respondingly slightly greater depth. 



" The remaining fact is that these leeches may be readily killed by 

 exposure to a temperature of 20° F. for a few hours. These 

 determinations were made in a small quantity of water and soft mud 

 and upon leeches shortly before living in a state of activity at higher 

 temperatures. Whether the same temperature would prove fatal to 

 leeches naturally hibernating is not definitely known. They may be 

 more resistant. Here is a source of possible but improbable error 

 that experiment only will determine (^). 



" The procedure recommended is as follows : About the beginning 

 of December, when the first thin ice starts to form, when it is certain 

 that the body of water in Carr Pond has attained its minimum 

 temperature and that the leeches have become dormant, the water is 

 to be drawn off as rapidly as possible until the level is lowered four 

 feet. It is to be maintained at this depth continuously for at least 

 five or six weeks during the coldest part of the winter, after which 

 the headgates will be closed in time to impound the melted snow 

 waters when the spring thaw comes. It is believed that a sufficient 

 margin of safety has been provided to cover the two factors of uncer- 

 tainty referred to above. Lowering the water level four feet should 

 include the greater depth at which the leeches might be expected to 

 winter at the more northern latitude. During the period chosen the 

 occurrence of several spells of zero, or near zero, weather and nightly 

 temperatures very generally below 20° F., may be expected. Under 

 these circumstances the exposed flats should be frozen hard to a 

 considerable depth and temperatures well below the fatal minimum 

 reach the imprisoned leeches. 



" The advantages of the plan are its simplicity, inexpensiveness, 

 and the ease with which it can be applied at one operation to the entire 

 lake. According to Mr. Welch it is feasible from the standpoint of 

 operation and open to no objections from that of engineering. 



^ Experiments conducted at my home during the early part of the winter 

 of 1919-20 determined that the minimum temperature lethal point is the same 

 for leeches taken from a condition of dormancy as had been determined previ- 

 ously for active leeches, that is, it is the same whether the teinperature has been 

 lowered rather rapidly or very gra;dually. In either case before dying they 

 become increasingly sluggish, quiescent and dormant and shrink in size, 

 probably through the loss of water chiefly in the form of mucous and nephridial 

 secretion. 



