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 Flukes. 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 Heterocotylea are 

 for the most part ectoparasites and the Asfidocotylea and Malacocotylea for 

 the most part endoparasites. The North American Heterocotylea con- 

 sist of five families, the Temnocephalida ; Tristomida, Monocotylida, Poly- 

 stomidte and Gyrodactylida ; 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. 



Gyrodactylid^ 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. 



Gyrodactylus. 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 Cyprinidae, especi- 



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