Figure 2.-- Ichthyophthirius multifilis, ciliated protozoan, the 

 "Ich" of aquarium and fish hatchery fish. 



*- * 



Figure Z,- - Myxobolus spore from white cysts. 



KiKure JT, — Vrllow grubs in lltwh of yt-Uow pcTch. (Courlray of 

 llif Nf>v York loiuuTvutiiiii I >Fpartiiirut ' 



Figure 4.-- Clinostomum marginatum "yellow grubs" in the flesh 

 of yellow perch. (Courtesy of the New York Conservation De- 

 partment). 



produces eggs which pass out in the feces 

 into the water. After a suitable period, 

 these hatch into a microscopic free-swim- 

 ming larva (miracidium) which has about 

 8 hours to find the right kind of snail. If it 

 succeeds, it burrows in and continues to 

 develop. It produces many of the next stage 

 larvae (cercariae) which burrow out of the 

 snail. To survive, cercariae have about a 

 day in which to contact a fish and burrow 

 into the tissue. In the fish, the worms may 

 migrate a short distance before reaching 

 their final site where they secrete a cyst wall 

 about themselves and become the "yellow 

 grubs", completing the cycle. 



Another type of fluke that is often noticed 

 is the "black grub" (fig. 6), a very small 

 larva enclosed in a black cyst about l/l6 

 inch in diameter. These may be the larvae 

 of any one of many trematodes but the most 

 common is known as Neascus. The life cycles 

 are similar to that of the "yellow grub." 

 Different fish, birds, and snails may be 

 involved. 



occur almost anywhere, including intern- 

 ally. If the cyst is removed and opened, the 

 larva can -be seen. The life history (fig. 5) 

 is typical for the group. When the fish is 

 eaten by a heron the larva is digested free 

 from the cyst and continues development 

 in the intestine of the bird. Most trematodes 

 develop to maturity in the intestine of the 

 host, but the "yellow grub" migrates to the 

 esophagus where it matures. The adult 



4. Gill flukes (monogenetic trematodes). 

 Most of these are microscopic, but one of 

 the larger ones, Discocotyle salmonis (fig. 7) 

 often occurs on the gills of trout and salmon. 

 It is about 1/4 inch long and attaches by 

 its large rear suckerlike attachment organ 

 to the gill of the fish. Sometimes these 

 flukes become so numerous that the fish 

 are seriously affected. Another very com- 

 mon monogenetic trematode, Gyrodactylus, 



