PARASITIC FLATWORMS 429 



is markedly distended. The distribution of various species is prob- 

 ably nearly concurrent with that of the particular hosts. 



North American cestodes are very imperfectly known and the 

 large part of the data available concerns species parasitic in birds 

 and mammals. Because of the lack of definite knowledge it has 

 been necessary to decide upon somewhat artificial limits, and the 

 synopsis has been made to include all cestodes reported in North 

 America from fresh-water hosts and all likely to have develop- 

 mental stages in fresh-water hosts even though such stages have 

 not yet been identified on this continent. Among the various 

 hosts from which tapeworms are reported the water birds are most 

 difficult to group correctly. Many of them visit both fresh and 

 salt-water bodies, and most of them feed at times and places on 

 terrestrial plants and insects either intentionally or incidentally. 

 Consequently, the source of a given infection is difficult to deter- 

 mine and some errors have no doubt been made in these cases; 

 yet, thanks to Ransom's careful and extensive work, avian ces- 

 todes are better known than those from any other host group in 

 North America. 



The data on North American cestodes are not only scanty but 

 also so indefinite as to be of little value in the attempt to prepare 

 a systematic outline of the group. Early references are to "ces- 

 todes" or "Taenia," and even in later years the same habit has 

 prevailed. Most existing records of the occurrence of tapeworms 

 in aquatic hosts cannot be referred to known genera. For these 

 reasons the appended synopsis must be presented with an apology. 

 Among the Proteocephalidae I have been able to depend on the 

 work of La Rue and for bird cestodes I have made free use of Ran- 

 som's monograph. Outside of these groups there is little definite 

 knowledge of the North American forms. 



KEY TO NORTH AMERICAN FRESH-WATER CESTODA 



1 (122) Adult worms; sex organs developed 7 



2 (7) Body simple, not divided into joints or proglottids. A single set of 



genital organs Subclass Cestodaria . . 3 



The few forms included here are often designated the monozootic cestodes, and sometimes 

 are regarded as a separate class intermediate in position between Trematodes and Cestodes. 

 In external appearance they resemble the flukes but are readily distinguished from them by 

 the entire absence of an alimentary canal. The internal structure is much like that of tape- 

 worms but the sexual organs are never duplicated. 



