REPRODUCTION AND ARTIFICIAL PROPAGATION OF FRESH-WATER MUSSELS. 157 



laboratory conditions it is found that a given fish may carry successfully a load of glochi- 

 dia so much in excess of what the same fish would ever be likely to carry in nature that 

 there is no reason why a single fish should not be made, under the conditions of artificial 

 infection, to do the work which a thousand fish perhaps could not do in the state of 

 nature. This has been from the first our main point of attack, and, with this in view, 

 we have studied the parasitism, first, by the infection of small lots of fish in aquaria 

 and, later, bv the infection of fish in larger numbers in a hatchery. Other points in the 

 life cycle, as for example the stage immediately following the parasitism, may be found 

 by later work to be places of wholesale destruction; we are convinced, however, that 

 there can be no other where the mortality reaches such proportions as it does when the 

 countless glochidia are spread upon the bottom and left to the chance that will bring 

 them in contact with the parts of a fish's body suitable for their parasitism. 



Throughout our experimental infections we have made use of small fish, usually 

 those under 6 inches in length, because such fish are more easily collected in numbers 

 and because we have not had proper facilities for the keeping of larger individuals. 

 Where small numbers of fish are used and each individual can be carefully watched, the 

 attainment of what may be termed an "optimum " infection in every case may be secured 

 with no great difficulty, and by following the methods practised by various investigators 

 ever since Braun (1878) and .Schmidt (1885), we have obtained unlimited material 

 whenever necessary. If the glochidia are placed in shallow dishes and in water just deep 

 enough to cover all parts of the fish, the latter will usually keep the water sufficiently 

 agitated to insure a proper suspension of the glochidia and tolerably constant results will 

 follow. 



It is very necessary that the glochidia be so distributed in the water as to come in 

 contact with the proper parts of the fish, and, in most cases, to guard against over rather 

 than under infection. Active fish, such as the rock bass {Ambloplites rupestris), and the 

 large-mouthed black bass (Micropkrus salmoides) , are very favorable for gill infections, 

 since they keep the water so well agitated that the glochidia hardly settle to the bottom 

 at all, while their strong respiratory movements draw the suspended glochidia con- 

 tinually against the gills. With fish like the crappie (Pomoxis annularis), which when 

 undisturbed move about quietly and whose respiratory movements are less vigorous, the 

 water must be stirred to keep the glochidia suspended, or be so shallow that the fish are 

 always near the bottom. The smaller gill slit of the crappie is another factor which 

 makes for a very light infection in fish under 2 inches in length, since the glochidia reach 

 the gills by way of the mouth and not from the opposite direction. For fin infections, 

 sluggish fish like the German carp {Cyprinus carpio) need little attention, and the darters 

 (Etiieosloma ca-rulemn spectabile), which habitually rest upon the bottom for considerable 

 periods, become quickly loaded with glochidia upon both fins and gills; although, as we 

 shall see, the latter fish appears to be particularly adapted for ridding itself of the entire 

 infection. 



In the account which follows, we are discussing the results obtained from the 

 infection of fish in small numbers and kept under careful observation in the laboratory. 

 85079° — Bull. 30 — 12 II 



