
AILMENTS AND DISEASES 

its host; and produces tiny red external, gill and throat 
ulcers into which it is pitted like a cancer. When present 
in numbers it is a dangerous parasite and has been found 
on fishes afflicted with what is popularly known as Twitters. 
It has a nearly circular ciliated body and on its lower side a 

FIG. 96 ; 
Trichodina pediculus Suctoral disc. 
a Hirudinid parasite. 
Greatly enlarged. 
ARACHNIA OR ARACHNID Parasites. This group 
includes the Mites, Ticks, etc., of which some of the aquatic 
species are discussed in the appendix to the Aquatic Insects. One family, 
Trachearia, contains strictly parasitic genera, and the 
Acharide and Hydrachnide have parasitic and predatory 
members which subsist largely on the freshwater fauna. 
The common form is the red Water-mite, Hydrachna 
geographica, Fig. 97. With five other families of this 
group parasitism is but slight and on the higher FIG. 97. Hydrachna 
geographica, an Arachnid 
Vertebrates. parasite. Enlarged, 

CrusTACEA OR CrusTAceAN Parasites. This 
group includes the so-called Fish Lice, small crustaceans known as Epizoa, 
and belonging to the families Lernadz, Caligide and Argulide. 
LeRN#D# AND Caticip#. The members of these families rarely 
occur as parasites on ‘the freshwater fauna, having those of saltwater as 
hosts; but some are brought into freshwater by marine fishes in their 

FIG, 98. Lernacera 
cyprinacea, a Crustacean 
parasite. Adult and free- FIG. 99 
swimming larva. Enlarged. ; 

Lernacera cyprinacea, attached 
to the gill of a young Large-mouthed Black 
Bass. Slightly enlarged. 
spawning migrations. One form, Lernecera cyprinacea, Figs 98 and 99, 
occurs in freshwater and is a frequent parasite on fishes. It attaches itself 
by peculiar sucker-tentacles to the gills, fins and surface, and is a very fre- 
quent parasite on river and pond fishes, of such size as to be easily seen. 
Arcutip®. These Copopods are known as Carp-lice, though they 
have almost all the freshwater fishes and amphibia as hosts. Of the three 
genera Argulus, Chonopeltis and Dolops, 42 species have been recognized, 
152 
