Ward. — Fish Parasites and Parasitic Diseases 221 



were free from any parasitic invasion and the average in- 

 fection was only three worms — one cestode and two nema- 

 todes. Yet in comparison with the size and reserve energy 

 of the host the infection was excessive, especially as the 

 single cestode, parasitic in the body cavity, distended the 

 body to an extreme limit. There was in fact at this point 

 and time a virtual epidemic among- the sticklebacks and we 

 picked up daily considerable numbers of dead and dying 

 fish at the surface of the water and along the shore. 



After having concluded thus a study of general condi- 

 tions we must also consider the special relations, since not 

 all parasites are of equal importance to the fish or to the 

 fish culturist. 



There are no general diseases produced by internal para- 

 sitic worms unless general weakness, loss of flesh and of 

 power of growth and reproduction be considered such. The 

 parasites are located in special places and usually produce 

 very definite effects. Nodules on the skin and in the gill fila- 

 ments are due to small encysted trematodes. Such a species 

 in black bass, pike, sunfish and perch is described by Os- 

 born ( l c »ll ). Some free trematodes also live on the gills 

 and in the mouth cavity. 



In the intestinal canal of fish Hofer records from Europe 

 49 different types of trematodes, 44 distinct tapeworms and 

 65 roundworms, and the number has been considerably in- 

 creased by the studies of recent years. The number reported 

 from North America is difficult to determine exactly, though 

 certainly much smaller. I estimate it to be about half as 

 great, but the reported forms are apparently much fewer 

 than those which are still unreported. 



Trematodes occur frequently in the eye of fish in Eu- 

 rope, causing blindness. While infrequent in nature this 

 trouble becomes common at times in fish ponds, not only 

 destroying the eye but being followed by the death of the 

 fish. The cause is found in minute larvae of certain flukes 

 which in the adult condition live in water birds. 



