Pearse—The Parasites of Lake Fishes. 
187 
The susceptibility of the host is an important factor in deter¬ 
mining the degree and frequency of parasitic infection. Proteo- 
cephalus pinguis was never found by the writer except in the pick- 
eral ; P. ambloplitis was found in a number of hosts. Fasten (1913) 
has studied a parasitic copepod which kills brook trout in great 
numbers, but will not live on the German brown trout. Howard 
(1914) cites similar instances among glochidia. Pish parasites 
may show considerable specificity for certain hosts and hosts may 
possess a varying degree of immunity. The black crappie, for 
example, appears to be immune to many parasites that attack 
other Cenfrarchidae. The largemouth and smallmouth black bass 
are closely related, but during the present investigations the latter 
always carried more parasites. The pumpkinseed was always 
more heavily infected than the bluegill. Perhaps some of these 
differences are due to differences in habitats, but some are un¬ 
doubtedly due to susceptibility. Furthermore, there is general 
similarity between the parasites of the Siluridae and certain Per- 
cidae which leads one to believe that infection may be limited 
at least in part by certain chemical substances which are present 
in or absent from the bodies of fishes. 
Surber (1913) has remarked on the remarkably small percent¬ 
age of fishes that carry glochidian. parasites in nature. Van 
Cleave (1919) showed that only half the species of fishes in a 
Michigan lake were infected with ancanthocephalans. Infection 
depends on so many factors and opportunities that it is to be ex¬ 
pected that parasites will frequently fail to reach their hosts. 
Most fish parasites do little harm to their hosts. Pratt (1919) 
states that nematodes are most injurious and that trematodes do 
the least harm. While his generalizations may apply^ to the para¬ 
sites of certain marine fishes, like the ,cod, the writer does not be¬ 
lieve they are generally applicable to the fresh-water fishes in the 
United States. The most injurious parasites (leaving out the 
Protozoa, which the writer has not studied) appear to be the 
larval tapeworms, which destroy liver tissue; aeanthocephalans, 
which cause ulcers in the wall of the intestine; and copepods, 
which suck blood from the gills. Glochidia, nematodes, and leeches 
are usually too few in numbers to do serious damage. Tapeworms 
and trematodes often occur in enormous numbers but do not appear 
to do much injury. The writer has never examined a dogfish that 
did not contain numerous tapeworms; yet all appeared to be in 
good condition. 
