SYNOPSES OF NORTH-AMERICAN 
INVERTEBRATES. 
XII. THE TREMATODES. 
PART IL— THE HETEROCOTYLEA OR MONOGENETIC 
FORMS. 
H: S. PRATT. 
THE order Trematoda was established in the year 1808 by 
Rudolphi, who included in it the following genera: Monostoma 
Zeder, Amphistoma Rudolphi, Distoma Retzius, and Polystoma 
Zeder. During the succeeding half century, when the greatest 
activity was shown in the description of new species of trem- 
atodes, numerous attempts were made by Von Nordmann, 
Dujardin, Diesing, Leuckart, and others to arrange them ina 
system of classification which would express their natural re- 
lationships. But lack of accurate information on their anatomy 
and development led, at one time and another, to numerous 
errors, such as the inclusion among trematodes of pentastomes, 
planarians, and leeches, and the description of larval forms for 
adult animals. Thus, it was not until the year 1858 that a 
system was constructed which was a satisfactory solution of the 
problem, and the one which is the foundation of the system in 
general use to-day. In that year P. J. van Beneden proposed 
the names “ Monogenea ” for those trematodes which develop ` 
without metamorphosis, and “Digenea” for those which 
develop with metamorphosis, the former being, for the most 
part, ectoparasites, and the latter, endoparasites. The great 
additions, however, which have been made in recent years to 
our knowledge of trematodes have rendered it increasingly 
difficult to use these distinctions satisfactorily, and conse- 
quently, in 1892, Monticelli proposed an entirely new system, 
in which trematodes are divided into three groups or suborders, 
the Heterocotylea, Aspidocotylea, and Malacocotylea, the first 
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