334 BULLETIN OF THE BUREAU OF FISHERIES. 



a manuscript of about 150 pages, which was subsequently turned over to the present 

 author. This has proved of great value on other occasions as well as the present, and 

 Gurley's original identifications and additions to the work of previous authors are 

 acknowledged in the following pages. 



The specimens and other material were derived from several sources. First, the 

 work of the biological station involves the handling of large numbers of fish, and several 

 of the regular staff, notably Mr. H. W. Clark, Mr. T. Surber, and Dr. A. D. Howard, 

 have saved such parasitic copepods as they found while examining the gills for glochidia. 

 These were generously turned over to the present author, who had also accumulated 

 a large number of specimens during the surveys of the mussel fauna of various regions 

 of the United States under the auspices of the Bureau of Fisheries. 



These collections were augmented during the present investigations by a careful 

 examination of all the preserved gills of fish in the possession of the biological station, 

 of the gills of live fish caught by the regular seining crew or brought to the station for 

 glochidial infection, and of a large number of dead fish caught by local fishermen. 



In these different ways, and including chiefly the waters of the Mississippi Valley, 

 the original list has been increased to 46 species, 10 of which are new to science; i of 

 Kr0yer's and i of Le Sueur's species have been rediscovered, and there have been added 

 the larvae of 4 other species in various stages of development. 



During the investigation it early became apparent that certain economic relations 

 existed between the copepod parasites and the mussel glochidia, which are also parasitic 

 on fish. Although the broad fact that parasitized fish do not take or hold glochidia as 

 well as the nonparasitized ones was observed early in the work at the station, neverthe- 

 less the existence of particular mutual relations between copepods and glochidia had 

 never been suspected. Of all the authors above mentioned Fasten is the only one who 

 has ever treated the copepods from an economic standpoint, and his excellent papers 

 deal chiefly with the artificial propagation of a single species. It is at once evident, 

 however, that the interrelations between the fish and the two kinds of parasites must 

 exert considerable influence upon the artificial propagation of mussels, as well as upon 

 an intelligent study of the parasitism of the copepods. Accordingly these economical 

 discussions are placed first in the present paper, and the description of the species is left 

 until the last. 



RELATIONS BETWEEN THE COPEPODS AND THEIR HOSTS. 



As has elsewhere been stated, both by the present author (Proceedings of United 

 States National Museum, vol. 25, p. 654) and by other investigators, it is not probable 

 that the copepod parasites of fresh-water fishes become under natural conditions a 

 serious menace to the life of their host. But it must be remembered that their presence 

 upon the fish is always injurious to the latter and can never be beneficial nor even 

 indifferent. 



I. There is a notion prevalent in certain quarters that a limited amount of dirt and 

 vermin is wholesome rather than harmful. It is needless to say that this is erroneous, 

 and that there is no truth also in the idea that a few of these creatures do their host 

 no real harm, but that a considerable number must be present in order to become really 

 injurious. Even a single parasite withdraws from its host enough blood for its own 



