4 University of Michigan 



to a strict localization of centers of infestation. If fishes bear- 

 ing parasites which are discharging embryos frequent a re- 

 stricted locality where suitable primary hosts occur in abund- 

 ance, such an area might easily become a localized center of 

 infestation. The individual fishes by chance or by choice 

 habitually using such a center of infestation as a feeding 

 ground w^ould acquire heavy infestations of Acanthocephala. 

 Other individuals feeding w-here infested primary hosts were 

 less abundant would acquire relatively few parasites. It might 

 be even possible for individuals to entirely avoid infestation 

 because of individual peculiarities in selection of food. 



Catostomus coinnicrsonii from the region under considera- 

 tion is the type host for N cocchinorhynchns crassus and for 

 Octospinifcr macilentus. The former of these parasites was 

 present in about 27% of the suckers examined and always 

 occurred in association with one or two other species of Acan- 

 thocephala. O. uiacilcntus appeared in 20% of the hosts. In 

 one instance it constituted the sole infestation while in the 

 remaining records it appeared along with both A'^. crassus and 

 P. btilbocolli. 



One specimen yielded two representatives of Hcliinorhynch- 

 iis thccatus. While these were both fully mature individuals 

 it seems probable that they represent an accidental infestation. 

 The fact that members of this species may find lodging in the 

 sucker and there reach sexual maturity indicates that there is 

 no physiological barrier to the establishing of this species as a 

 normal parasite of the sucker. The only apparent explana- 

 tion of its general absence in the sucker must lie in the fact 

 that neither primary nor intermediate hosts of E. tJiecatiis 

 enter into the food supply of this fish. 



The sucker is by far the richest in acanthocephalan fauna 

 of any of the fishes found in this region. One of its parasites. 



