58 Roosevelt Wild Life Bulletin 



the gill-filaments and suck the blood of the fish through their delicate 

 walls. A heavy infection of this sort might easily result in such a 

 serious loss of blood by the fish that it would languish and die. The 

 lungs of frogs almost always contain flukes that sometimes quite fill 

 them. The air-passages and lungs of birds, turtles, snakes, and other 

 aquatic vertebrates are also often infected. 



All parasitic worms have very great reproductive powers, and 

 when the conditions accompanying their embryonic and larval develop- 

 ment are unusually favorable, as occasionally happens, they may 

 increase in number to such an extent that a parasitic epidemic follows 

 and thousands of animals of the species afifected may die. Such 

 epidemics occur among fish from time to time, and are sometimes 

 familiar to fishermen because of the large number of dead fish they 

 find strewn along the beach. Where the fish which are thus destroyed 

 belong to species having commercial importance it is evident that the 

 economic loss may be great. 



Where an animal is but slightly infected with animal parasites 

 they may not apparently aflfect its health or general well-being, but 

 it is probable that any considerable infection, while not perhaps 

 sufficient to afifect the activities of the host, would reduce its repro- 

 ductive powers and prevent it from bringing into the world the 

 normal number of young. Parasitic castration, more or less com- 

 plete, is a matter of considerable importance in the animal world, 

 and has been frequently observed among Crustacea and other inver- 

 tebrates. The parasitized animal may be affected either by the 

 destruction of the tissue of the sex glands by the parasites, or by 

 being so weakened by them during its developmental period of life 

 that the sex glands fail to develop or to function normally, and the 

 animal's mating instincts remain inactive. It is probable that in the 

 case of fish and other vertebrates the effect of parasitism on repro- 

 duction is similar. Definite information, however, is lacking, and 

 observations directed to the matter would be of great value. 



Parasites sometimes affect the commercial value of food fishes 

 even when apparently they have no influence upon their health or 

 their ordinary habits of life. Perch, rock bass, sunfish and other 

 food fish in Oneida Lake are frequently caught, especially in the 

 late summer and fall, in which good sized cysts are found imbedded 

 in the muscles, often just beneath the skin, in the gills, fins or in 

 other parts usually more or less superficial. Each of these cysts 

 contains a Trematode worm ranging from 4 to 10 mm. in length, 

 and is usually very noticeable. Although the worms are entirely 

 harmless to man, and would simply be digested with the flesh of the 

 fish if eaten, wormy fish are usually thrown away by fishermen 

 and cannot be sold in the market. A popular account of the relation 

 of the worm parasites to fishes and the influence of these parasites 

 on their food value has been published as a part of the present 

 study, as : Parasites of Fresh- water Fishes, U. S. Bur. Fisheries, 

 Econ. Cir. No. 42, pp. 1-8, 1919. 



Many other species of fish also may harbor cysts containing larval 

 Trematodes, Cestodes or Nematodes in their muscles and thus lose 

 a part or all of their commercial value. 



