Leeches 27 



in rate, amplitude, and vigor as determined by a great variety of 

 conditions. It may occur during feeding, when probably it has the 

 additional utility of facilitating the filling of the caeca. Leeches are 

 able, however, to live under nearly or quite anaerobic conditions 

 (Putter, '07). 



As in most aquatic invertebrates the rate of metabolism, and conse- 

 quently of the activities of leeches, is a function of the temperature, 

 increasing as the water becomes warmer to an optimum not yet 

 accurately determined, and diminishing as it cools. This is probably 

 another reason why these leeches seek the shallows where the water is 

 warmer during the greater part of the year and especially during 

 the spring. In the autumn as the water cools the leeches remain 

 much of the time quiescent and probably seldom feed. When the 

 water temperature approaches 40° F. they become extremely sluggish 

 and seek winter quarters, and at 39°, the temperature of greatest 

 density of fresh water, they become practically dormant and nearly 

 insensitive but will respond by slow movements if pinched with a 

 forceps, pricked with a needle, or exposed suddenly to a strong light. 

 At this time they appear to be somewhat reduced in size and it may 

 be that, like earthworms and many other animals at low temperatures, 

 they have lost water, but this has not been determined. 



In the vicinity of Philadelphia Macrohdella may be found through- 

 out the winter on the tidal flats of the Delaware River or on the 

 shores of ponds beneath logs and stones partly buried in the mud 

 and seldom in water more than eighteen inches or two feet deep. 

 In such places they may or may not have burrowed two or three 

 inches into the mud and appear quite dormant until, with even a 

 slight rise in temperature, they are stimulated to show some activity. 

 Leeches of this and other species are frequently found in nature in 

 a lethargic state in the unfrozen mud immediately beneath the ice or 

 in actual contact with or even imbedded in the latter. 



That they are quite unharmed by freezing temperatures, I have 

 many times demonstrated in laboratory experiments in which the 

 water in the vessel containing them was wholly or partly frozen ; and 

 they have lived thus for several days, showing no indication of injury 

 whatever when gradually thawed out. Further experiments, con- 

 ducted both during the summer upon active leeches and during the 

 winter upon individuals naturally dormant, have fixed with sufficient 

 accuracy for present purposes the lethal point to cold. It has been 

 found that a temperature of 20° to 22° F. was invariably fatal even 

 when the exposure lasted for only three to eleven hours and the thaw- 

 ing out was very gradual. These experiments were conducted by 

 placing two or three healthy leeches at a time in basins or tight boxes 

 about five inches deep, either with water only or with three or four 

 inches of wet mud, sand, stones, and bits of bark in and beneath 

 which the leeches were buried to varying depths. The vessels were 

 then placed either on ammonia coils in a freezing room, in ice and 

 salt mixtures, or during the winter out of doors. The air tempera- 

 ture and the temperature of the contents of the vessel were deter- 

 mined, the former usually by an ordinary house thermometer and the 



