2 PROCEEDINGS OF THE NATIONAL, MUSEUM. vol. 61. 



MATERIALS. 



The materials upon which this study was carried on were taken 

 from various locations in the area under consideration. Individuals 

 were first encountered in the intestine of Roccus chrysops at New 

 Boston, Illinois, June 30, 1919. By far the greatest part of the collect- 

 in<i: was done during July and August on Lake Pepin at Lake City, 

 Minnesota. A few specimens of the host from Lake Pokegama at 

 Pine City, Minnesota, were also examined and found to be infested 

 with this parasite. A single individual of the white bass taken from 

 the Mississippi River at Fairport, Iowa, bore a moderate infestation of 

 Allacanthochasmus. A considerable number of prepared whole 

 mounts of these trematodes were very kindly turned over to the writer 

 by Prof. A. S. Pearse who collected the specimens in the course of his 

 investigations at Lake City, Minnesota, during the summer of 1920. 

 From one host individual of this collection a number of liver cysts 

 contained egg-bearing trematodes of the same species as those en- 

 countered in the intestine of the same host. The writer is unable to 

 determine how these individuals reached this unusual place. 



In September, 1913, the writer made rather extensive collections 

 of fish parasites at Sandusky, Ohio. Subsequently only a portion 

 of the material of this collection was worked over and the species 

 determined. Eecent examination of some of the previously unde- 

 termined trematodes from Roccus chrysops has revealed the fact that 

 they are of the same species discovered in the white bass of the 

 Mississippi Basin. It would thus seem that the geographical dis- 

 tribution of this trematocle is probably coincident with that of its 

 host or at least the parasite occurs through much of its range. 



Members of the genus Allacanthochasmus seem to be remarkably 

 adapted to their host for infestation is apparently specific. Of the 

 hundreds of fishes examined, representing all of the usual species in 

 the regions investigated, not a single specimen of Allacanthochasmus 

 was encountered in any host other than Roccus chrysops. Every speci- 

 men of the white bass examined carried an infestation and in many 

 instances the parasite was present in large numbers. The worms of 

 this species are very minute and apt to escape observation even 

 though they may be present in great numbers, for they become ob- 

 scured in the intestinal material. 



Mature individuals show extreme variability in size. The differ- 

 ences are so marked that one might feel inclined to consider the 

 largest and the smallest as specifically distinct were it not for the fact 

 that these extremes are united by a series of fine gradations. 



In spite of the fact that body musculature is not excessively de- 

 veloped, these worms both in the living and the preserved condition 

 display the results of unusual powers of general and localized con- 



