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162    Wisconsin Academy of Sciences, Arts, and Letters.
summer, and a maximum was reached during hibernation. There
is a great need of more information concerning the seasonal prev-
alence of the parasites of all aquatic animals.
  Of the factors that control the occurrence of fish parasites there
is also a dearth of knowledge. Hausmann (1897) states that when
fishes eat little on account of cold or heat, parasites are few; and
he assigns an important role to temperature as a factor in parasitic
infection. He also points out that most parasites enter fishes with
food. Ward (1909) stresses the fact that parasites respond to
changes in the habits of their hosts to such a degree that their
presence or absence furnishes evidence of particular habits. Pratt
(1919) affirms that epidemics of fish parasites are apt to occur
when the water is warm and that small inclosed bodies of water
harbor more parasites than those of larger size because fishes can-
not escape by migration.
  The present paper describes the results of statistical studies on
the occurrence of fish parasites in different types of lakes. The
writer was led to make such studies in attempting to discover why
fishes fail to grow much in certain bodies of water while they may
attain large size in other bodies near by. It seemed desirable to
learn whether particular lakes showed specificities in regard to the
numbers and kinds of parasites present and whether there is cor-
relation between the presence and size of particular fishes and
the presence or absence of parasites. The work began in 1917 and
was at first confined to the yellow perch. Observations were made
on specimens from sixteen lakes on three different river systems.
Later, more extensive observations were made on five different
types of lakes where the parasites of all available species of fishes
were studied.
  In studying fishes for parasites they were always examined
while fresh, as it was found that results from old or preserved
fishes were of little value. The skin, fins, mouth and gills were
first scrutinized; then the specimen was opened from vent to throat,
and the visceral organs were examined. The contents of the ali-
mentary canal were stripped out on a glass plate and the canal
itself was opened from end to end with scissors. The food and
faecal matter were carefully teased across under a binocular micro-
scope. The number and location of all parasites was entered on a
form sheet, one sheet being used for each fish examined. Parasites
were placed in corrosive sublimate solution and alcohol. Later
they were stained and mounted. In this paper all measurements