Leeches 35 



an artificial preservative, especially as it appeared to be so much less 

 effective than blood previously used at Philadelphia. Inquiry brought 

 the assurance that it was " pure heart blood " of the beef entirely 

 without preservative or treatment of any kind. Some experiments 

 conducted in aquaria in the laboratory showed that the leeches would 

 follow a clear trail of this blood and that they would eat it, though 

 with much less eagerness than the fresh blood of frogs. 



Before a supply of fresh blood was arranged for, some laboratory 

 experiments were conducted on several substances in the hope of 

 finding a satisfactory substitute for use as bait. Among the sub- 

 stances tried were dried blood (commercial fertilizer), beef extract 

 (Armour's), musk, urine, and oil of anise. By far the most active 

 response was made to the beef extract, which was readily traced 

 across the aquarium to the finely drawn out end of a tube from 

 which it was slowly diffusing. It was also imbibed to a limited 

 extent. In about half of the experiments with dried blood this also 

 induced a positive reaction but in no case did the leeches appear to 

 eat it. To the musk there was only a very weak response, and to the 

 urine and anise oil there was no appreciable positive reaction. 



Gathering the Leeches and their Egg-cases. Up to a half- 

 century ago the use of blood-sucking leeches for medicinal purposes 

 continued of sufficient generality that, in addition to importations 

 from Europe and the product of American leech farms, large num- 

 bers of our native wild leeches {Macrobdella) were utilized. Especi- 

 ally in the environs of Philadelphia several collectors maintained a 

 considerable and profitable business. The leeches were obtained 

 chiefly from the ponds of Montgomery, Bucks and Berks Counties, 

 and on the tidal flats and marshes on both sides of the Delaware 

 River. After the decline of this business Macrobdella decora in- 

 creased greatly in numbers in this region. During the progress of the 

 World War, with other supplies cut off, there arose a slightly revived 

 demand for native leeches, especially among the foreign population of 

 New York and other large cities, and some inquiries were made by 

 pharmacists and chemists for leeches as a source of supply of hirudin 

 for use in surgery. During my visit to Carr Pond some interest 

 was aroused among a few of the more commercially-minded boys at 

 one of the camps, and they were influenced to gather some of the 

 leeches and dispose of them at a fair price to a dealer in New York 

 City. If it could be done without conflicting with the ideals and 

 discipline of the camps this practice might be largely extended, and 

 in the aggregate a considerable number of leeches got rid of. 

 Probably much more could be accomplished in this way than if men 

 were employed to gather them, and without any expense to the com- 

 missioners. No better opportunity for collecting them can be 

 devised than that presented during the swimming periods at the 

 camps, the swimmers themselves serving at the same time as 

 animated baits and traps. Were this plan carried out consistently at 

 all of the camps for a period of years it is certain that a very material 

 reduction in the numbers of the leeches would be witnessed. General 



