Figure 10.-- Pis cicola salmositica, a leech or "blood-sucker" 

 (after Meyer). 



Parasites of the Viscera 



1. Protozoa. Internal fish protozoa are 

 usually not seen by the casual observer 

 except for the myxosporidian cysts dis- 

 cussed in previous sections. 



2. Flukes. "White grubs" are sometimes 

 found in the visceral organs. These are 



somewhat similar to the "yellow grubs" 

 discussed previously, and are not harmful 

 to man. Adult flukes of fish occur in the 

 intestine and stomach, but are usually small 

 and not seen. 



3. Tapeworms (cestodes). Two forms of 

 these are seen in fish. One, the adult, 

 lives in the intestine of fish. It is flat like 

 a tape, and often several inches long. It 

 is this stage that is seen when the intestine 

 is accidentally cut or torn during cleaning 

 of the fish. The other form is the larva 

 (plerocercoid), which may or may not be 

 the same species as the adult seen in the 

 fish. Larvae are smaller, nonsegmented, 

 and may occur among the internal organs 

 as well as in the flesh of the fish. Cal- 

 careous corpuscles (round concretions of 

 lime) can be seen microscopically in all 

 tapeworm larvae; this is sometimes a very 

 helpful characteristic for identifying a lar- 

 val tapeworm parasite. The adult from 

 fish is never harmful to man, but some of 

 the larvae, if eaten raw, can develop in 

 man. We have previously discussed the 

 best known one in this group, the broad 

 fish tapeworm (p. 7). 



One of the most noteworthy fish tape- 

 worms is the bass tapeworm, Proteocephalus 

 ambloplitis (fig. 13). The larvae (plerocer- 

 coids) often cause sterility in black bass. 

 The adult lives in the intestine of black 

 bass and produces large numbers of eggs 

 which pass out with the feces of the fish 

 into the water. The small larva in the egg 

 will develop to the next stage larva (pro- 

 cercoid) if eaten by the proper kind of 

 copepod. After development in the copepod, 

 this larva invades small fish which eat the 

 infected copepods. The larva is freed in the 

 intestine of the small fish, burrows through 

 the intestinal wall, and wanders among the 

 internal organs. When large numbers of 

 these larvae (plerocercoids) are present 

 they cause considerable damage to the fish. 

 Small bass, once infected, will retain the 

 larvae for a long period of time. Larger 

 bass accumulate larvae by eating small 

 fish that have recently fed on infected cope- 

 pods. Larvae, if still in the stomach or 

 intestine of the small fish when eaten by the 

 larger fish, will migrate through the intes- 

 tinal wall and into the visceral cavity of 



