Worm Parasites 59 



Of the four classes of vermian parasites that infest fish and other 

 aquatic vertebrates perhaps the most important from an economic 

 standpoint are the Nematodes and the Acanthocephala. These worms 

 are slender, cylindrical animals, ranging from mifcroscopic size to 

 several centimeters in length, which sometimes infest their hosts in 

 such large numbers that they either kill them outright or reduce their 

 strength and vitality sufficiently to place them at a fatal disadvantage 

 in the struggle for existence. The Nematodes, or threadworms, are 

 usually found either in the digestive tract or in cysts embedded in 

 the muscles or fat or attached to the peritoneum or to a mesentery. 

 In the former case the worms are active adults, which usually range 

 more or less freely in the portion of the digestive tract in which they 

 live, sucking the blood or absorbing the vital fluids of the host and 

 producing their eggs or young. They undoubtedly often injure the 

 host by lacerating the intestinal wall and draining its vitality, and 

 perhaps also by enabling bacterial or protozoan parasites to gain a 

 foothold in its tissues and blood. The hookworm, which has in the 

 past few years obtained such an unpleasant notoriety in the Southern 

 States of this country, belongs to this group of parasitic worms. 



Encysted threadworms are in their larval stage of development, 

 and in this condition they lie dormant and inactive in their hosts 

 without apparently doing them any harm. The cysts, however, like 

 those of the flukes already mentioned, often affect the market value 

 and sale of food fish, as purchasers of fish in a fish market will not 

 usually buy fish which they see to be wormy, even when the worms 

 are harmless, as they always are, and do not decrease the food value 

 of the fish. 



The Acanthocephala or spiny-headed worms live in the intestines 

 of their hosts and are often very numerous in aquatic vertebrates. 

 They resemble threadworms in appearance but differ from them, 

 among other things, in possessing a prominent, more or less retractile 

 projection, the so-called proboscis, at the anterior end of the body, 

 which is covered with recurved hooks by means of which they attach 

 themselves to the intestinal wall of the host. When they infect an 

 intestine in large numbers, as they frequently do, the laceration of 

 the intestinal wall by their spiny proboscides undoubtedly causes a 

 severe injury to the host and may cause its death. 



The Cestodes or tapeworms are very important parasites of aquatic 

 vertebrates. The adult worm lives in the intestines of its host; the 

 larval worm may be found in almost any organ of the body and in 

 the body cavity, usually enclosed in a cyst, but in the case of certain 

 species not encysted. Tapeworms are much the largest vermian 

 parasites of aquatic vertebrates, some of them measuring 30 cm. or 

 more in length, and are often fatal to fish containing them. The 

 cysts are sometimes very common objects in the flesh of many of the 

 smaller food fishes. 



The Trematodes or flukes are probably the least harmful of the 

 internal vermian parasites of aquatic vertebrates. The worms are of 

 small size, usually measuring a few milhmeters in length, and in most 

 cases are not found in large numbers in a single organ of the host. 



