4i8 A HISTORY OF FISHES 



does little harm except in the ponds of hatcheries, where the 

 fish are crowded together. 



Internal parasitic worms of one kind or another often occur 

 in most marine and fresh-water fishes, and include Trematodes 

 or Flukes, Tape Worms, Round or Thread Worms, and 

 Thorn-headed Worms. Flukes (Trematoda) are most common 

 in river and lake fishes, some occurring in the adult form, either 

 as ectoparasites on the gills or skin, or as endoparasites in the 

 alimentary canal, while others in the pre-adult stage become 

 encysted in the tissues or in the body cavity. Thus, sometimes 

 the fish is the final host of the fluke, but in other cases it is only 

 a temporary or intermediate host, and the development of the 

 parasite is completed within some fish-eating vertebrate. Roach, 

 Dace, Bream, and other coarse fish are sometimes found with 

 the body covered with small black spots, and examined under a 

 microscope each of these is seen to consist of a mass of pigment 

 surrounding a larval Trematode. No harm appears to be 

 done to the fish, and heavily spotted individuals are as healthy 

 as those which are uninfected. 



As with the Flukes, so with the Tape Worms (Cestoda), the 

 fish is sometimes the final host, the worm passing the earher 

 stages of its development in some smaller animal preyed upon 

 by the fish, and sometimes only the intermediate host. Two 

 genera, of which one (Ligula) occurs in various fresh-water 

 fishes, and the other (Schistocephalus) is very common in the 

 Sticklebacks, grow to a very large size as lar\^al forms in the 

 body cavity of the fish, and become adult in the intestine of 

 various fish-eating birds. In certain seasons nearly all the 

 Sticklebacks living in a given locality have been found to be 

 infected with this parasite, which fills the abdominal cavity 

 to such a degree that the host appears unusually plump or is 

 even swollen like a miniature balloon. In North America, 

 Tape Worms forty centimetres in length, attributed to the 

 genus Ligida, have been taken from Suckers [Catostomidae) only 

 ten centimetres long, the weight of the parasite being more 

 than one-quarter that of the fish. The worms lie quite free in 

 the body cavity, and although they do not move about and 

 have no suckers or hooks to cause definite injury, they do 

 considerable damage by crowding the organs of the fish into 

 unnatural positions, as well as by absorbing the serous fluid. 

 It is of some interest to note that Ligula is considered a delicacy 

 in Italy and the south of France, where it is known as " Maccaroni 

 piatti" and "Ver blanc" respectively. A Tape Worm known 



