PARASITES OF FISHES 49 



and viscera. As far as we know, the species of these worms found in 

 fishes are harmless to man. They usually occur in the viscera, so most 

 of them are discarded in dressing the fish. 



Spiny-headed worms (Acanthocephala) often occur in the intestines 

 of fishes. They resemble roundworms in size, but differ in other ways. 

 They possess an anterior proboscis covered with minute hooks, by 

 means of which they attach themselves to the wall of the intestine. 

 Often large numbers may infect fishes and cause considerable damage. 



The larval stages of a number of parasitic worms live in the flesh 

 and skin of fishes. One common species is the yellow grub (Clinosto- 

 muni marginatimi) , which is a larval form of one of the flukes or flat- 

 worms (Trematoda) . This worm appears as a yellowish swelling in or 

 just under the skin of many fishes, such as rock bass and perch. The 

 adult worm lives in the mouth of various fish-eating birds like the 

 great blue heron. The eggs pass into the water and hatch into free- 

 swimming larvae {imracidia) , which enter snails. Inside the snails 

 these larvae reproduce and eventually leave the snails as tiny free- 

 swimming forms (cercaria) , which on finding a fish burrow into the 

 skin and encyst. There they remain until the fish is eaten by a bird. 

 In the bird they complete their development and become adult worms. 

 They are harmless to man and do httle damage to fishes. They are 

 practically impossible to eradicate because it would be necessary to 

 destroy completely all fish-eating birds or all snail hosts. 



Another very common parasite in most northern fishes is the black 

 grub (Neasctis) , which causes black spots in the skin of perch', northern 

 pike, and many other fishes. This worm is another larval flatworm or 

 fluke. The adults live in the mouths of kingflshers, from which the 

 eggs pass into the water. As in the case of yeflow grubs, the first larval 

 stages find their way into snails and eventually pass into a fish, where 

 they form black cysts in the skin, and must be eaten by a kingfisher 

 to complete their development. Frequently a fish may be so heavily 

 parasitized as to be fairly covered with these black cysts. This parasite 

 will not infect man, and the cysts can be removed by skinning the 

 fish. Fishes which spend most of their lives in open water and do not 

 come near the shores where snails live are least infected with this 

 parasite. In Lake Superior, where snails are relatively rare, the parasite 

 is not common. 



Some tapeworms have larval stages which form cysts in the muscles 

 of fishes. The most common of these are the several species of 

 Triaenophorus, which form the elongate yellow worms found common- 

 ly in the back muscles of the tullibee and the white fishes of our inland 

 lakes. The adult tapeworms live in the intestine of the northern pike. 

 The eggs are released into the water and develop into free-swimming 

 larvae which enter copepods (Cyclops) . When these minute crusta- 



