STUDIES ON THE DIGENETIC TREMATODES. 391 
Studies on the Structure and Classification of 
the Digenetic Trematodes. 
By 
William Nicoll, M.A., D.Se., 
of the University of St. Andrews. 
With Plates 9, 10. 
(© Tue following work was commenced about the middle of 
1907 at the Gatty Marine Laboratory, St. Andrews, under 
the Carnegie Trust Research Scheme. The greater part of it 
was done privately in the Pathological Laboratory, University 
College, Dundee, and it was concluded at the Marine Biolo- 
gical Laboratory, Millport, under a Government (Royal 
Society) Grant. 
My best thanks are due to very many friends for assistance 
in various directions, and in particular to Professor McIntosh, 
of St. Andrews, for continual encouragement and suggestions, 
and to Professor Sutherland, of University College, Dundee, 
for granting me the privilege of working in his laboratory 
and for helping to elucidate several pathological difficulties. 
In this paper an attempt has been made to allocate each of 
tne forms dealt with to its approximate systematic place—a 
matter of considerable difficulty in many cases. At the 
present time a classification of the digenetic Trematodes in the 
true sense of the term can hardly be said to exist. ‘T'v 
attempt to define a higher systematic unit than the genus is 
attended with much risk. A considerable number of sub- 
families have certainly been created, but few of these consist 
of more than two or three genera, and are really at most to 
be considered as indications according to which classification 
may be expected to proceed. The formation, however, of 
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