368 FRESH-WATER BIOLOGY 



dish of suitable size and examining the contents of various organs. 

 The parasites usually betray their presence by sluggish move- 

 ments of the body which make even minute objects conspicuous 

 in a mass of debris. A watchmaker's lens held in place at the eye 

 by a spring is of service in recognizing and sorting out the smaller 

 forms, and long bristles or a camel's hair pencil are useful in pick- 

 ing out the forms for study and preservation. Doubtful objects 

 should be examined under a higher magnification whereupon the 

 firm, definite outline of a parasite enables the student to distinguish 

 it even when motionless from partly digested fragments of food, 

 blood clots, or other foreign bodies of similar size and texture. 



Parasitic flatworms may be kept some hours in weak normal salt 

 solution for examination or even in tap water, but deteriorate so 

 that for careful study material should be preserved as soon as 

 possible. For preservation an aqueous solution of corrosive sub- 

 limate is most satisfactory, and the precise method of handhng 

 suggested by Looss gives results well worth the extra time and 

 trouble because of the greater ease with which future work may be 

 carried on. Because of the great similarity in external form be- 

 tween different types, a determination can be safely reached only 

 after a worm has been stained and mounted in toto, or sectioned 

 in case of large and opaque specimens. 



The parasitic flatworms have received relatively Httle attention 

 in North America; it is consequently a difficult matter to prepare 

 a synopsis that is of value to the student, for from our knowledge 

 of the group in other parts of the world it is safe to assert that 

 the known forms do not constitute more than a small fraction 

 of those that actually exist on this continent. Another difficulty 

 which presents itself is the impossibihty of defining clearly the 

 Hmits of the topic. I have endeavored to include in the key all 

 North American parasitic flatworms thus far recorded from fresh- 

 water animals whenever the record permits of any reasonable 

 interpretation. I have omitted a few records so brief or indefinite 

 that a diagnosis was impossible. There is included also a consid- 

 erable number of parasites from distinctly land animals, the life 

 history of which is certainly bound up with stages parasitic in the 

 fresh-water fauna. On the other hand I have omitted all clearly 



