
AILMENTS AND DISEASES 

meals of other animals, attaching themselves to, or only accompanying, 
their more vigorous hosts. 
The science of Helminthology has determined that many of the para- 
sites live at various stages of their existence in widely different animals. 
This particularly applies to those of fishes, which more than others are 
subject to parasitism, not only in the number which they harbor but also 
in the frequency with which this occurs. Each genus is subject to a num- 
ber peculiar to itself as well as some common to all. These inhabit various 
parts of the body, the skin, connective tissues and muscles; the heart, 
liver, respiratory and digestive organs, either free or encysted. The most 
of them, however, exist in the intestines and alimentary canal or in the 
gills and on the surface. They are sometimes harmless but more often 
injurious, as their progeny may be so numerous as to tunnel in all direc- 
tions until the whole organ or part of the tissue which they inhabit is little 
more than a sac of microscopic worms. 
Fishes acquire internal parasites with their food, while those which 
affect them externally, are usually free-swimming at some stages of their 
existence. These belong to different groups of the lower animals, of which 
some of the common North American forms will be enumerated, for the 
further identification of which the reader is referred to the authorities 
mentioned in the Bibliography appended hereto. 
TREMATODA OR FiLuKEs. ‘The members of this group are small para- 
sitic flatworms with unsegmented flattened or cylindrical unciliated bodies, 
usually having anterior mouth-openings, bifurcated intestine and without 
anal opening, which attach themselves to their host by the means of 
suckers or hooks, or both, and live upon their juices. The Trematoda 
are classed in three groups or sub-divisions, of which the Hererocotylea are 
for the most part ectoparasites and the Aspidocotylea and Malacocotylea for 
the most part endoparasites. The North American Heterocotylea con- 
sist of five families, the Vemnocephalide; Tristomide, Monocotylidea, Poly- 
stomide and Gyrodactylide; divided into 8 families and 52 genera, mostly 
parasitic on Vertebrates and principally in marine animals, but some 
species have freshwater fishes and amphibia as host, of which one genera, 
the Gyrodactylida, will be particularly mentioned. 
GyropacTyLip# This family includes the genera Gyrodactylus and 
Calcostoma, the former having double or more numerous prehensile hooks, 
the latter a single horny structure at the margin of the caudal sucker. 
Gyropacty.Lus. This parasite is found on the gills of freshwater 
fishes in numerous specific forms, almost each species supporting a differ- 
ent form, and sometimes two or more on the same gill. The most com- 
mon species, G. elegans; Fig. 80; infests the gills of Cyprinide, especi- 
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