LIVER 



STOMACH 



SWIM BLADDER 

 KIDNEV 



DORSAL FIN 



ANAL FIN 



INTESTINE 

 HEART 



'ENTRAL FIN 

 PVLORC CAECA 



Figure 1.- -Drawing of Lepomis cyaneltus, (from Hile, 1960, U. S. Fish and Wildlife Service. Fishery Leaflet 132). 



FRESH-WATER PARASITES 

 Parasites of the Body Surfaces and Gills 



1. Fun^. White cottony growths on wounds 

 or ulcerations are usually fungi of the 

 genera Saprolegnia and Achlya. The filaments 

 can be seen easily under the microscope. 

 The fungus often develops as a secondary 

 infection following wounding or ulceration 

 due to other agents. 



2. Protozoa. These are the smallest ani- 

 mals. They consist of a single cell, and 

 nearly all are too small to be seen with- 

 out a microscope. Some protozoa fornn 

 \vhitish cysts containing many spores. Such 

 cysts are large enough to resemble worm 

 larvae superficially. They may be found 

 in many organs including the skin and gills. 



Often in aquaria and fish hatcheries, a 

 condition develops which is known as "Ich" 

 or "white spot." The fish are seriously 

 ctffected, and many little white spots can 

 be seen - some of them moving slowly over 

 the fish. These are individual protozoa, 

 Ichthyophthirius muUifilis, the largest protozoan 



to be found on fish. Under the microscope 

 the most striking characteristics that can 

 be seen are the constantly beating hair- 

 like cilia, which completely cover the para- 

 site (fig. 2.). 



All other protozoa that live on the skin 

 of fish are very small in comparison to 

 Ichthyophthirius. Although some of them are 

 serious fish disease agents, none is harm- 

 ful to man. 



Myxosporidian cysts containing many 

 spores (fig. 3) are whitish and usually 

 large enough to be seen easily. They are 

 found in many organs including the skin 

 and gills. 



3. Internal flukes (digenetic trematodes). 

 These small worms are flat, leaflike forms 

 from 1/250- to l/S-inch long with two 

 suckers. The trematodes most frequently 

 seen are larvae ("grubs") encysted in the 

 skin or flesh. The largest of these larvae 

 is the yello^v grub, Clinostomum marginatum (fig. 

 4). The cyst is yellowish, about 1/8 inch 

 in diameter, and is usually seen in the 

 gills and at the bases of the fins but nnay 



