188 Wisconsin Academy of Sciences, Arts, and Letters. 
The migrations of fresh-water fishes doubtless afford them op¬ 
portunity to acquire and shed parasites (Ward 1909): The fishes 
that travel most and invade the greatest variety of habitats in 
general have the most parasites. 
Seasonal changes doubtless have a marked effect on certain fish 
parasites. It was found that perch have most parasites in spring, 
although some species of parasites were more abundant at other 
seasons. Ward (1909), Marshall and Gilbert (1905) and Van 
Cleave (1916) have also made observations on the abundance of 
parasites at various seasons, but such information is as yet too 
limited for generalization. 
Hausmann (1897) thought that perch had few parasites when 
little food was eaten on account of low temperature. In the 
writer’s experience perch have not been found to refrain from 
eating during winter and they have more parasites in winter than 
in autumn or summer. Pratt (1909) says that epidemics of 
trematodes are likely to occur when the water is warm. The writer 
has found no evidence that this is the case, at least in fresh-water 
fishes. 
Some parasites are apparently limited to particular drainage 
systems. Phyllodistomum superhum Stafford was quite common 
in the urinary bladder of perch in several lakes on the St. Law¬ 
rence drainage but was never observed in a single one of the sev¬ 
eral thousand perch examined from the lakes on the Mississippi 
drainage. Perhaps this trematode entered the St. Lawrence drain¬ 
age after the Mississippi separated from it. Other cases of this 
kind probably occur but too few specimens have yet been examined 
to demonstrate them. 
Contrary to Pratt’s (1919) assertion, the size of a lake does not 
appear to be correlated with the degree of parasitic infection of 
its fishes. The density of the population may sometimes be of im¬ 
portance. However, the fishes of Lake Mendota, which has the 
densest population of any of the five lakes studied extensively by 
the writer, has the fewest parasites and the fishes in the depth of 
Lake Michigan, where the population is scanty, have many: There 
is, in general, no direct relation between number of fishes and num¬ 
ber of parasites. 
The habitats of fishes are of course important in their relation 
to parasitic infection. While a variety of habitats is desirable for 
the growth of fishes, it gives opportunity for acquiring more para¬ 
sites. A wide range also gives more opportunities for acquiring 
