REPRODUCTION AND ARTIFICIAL PROPAGATION OF FRESH -WATER MUSSELS. 1 67 



In any attempted infection of fish in large numbers, careful tests should first be made 

 upon a few fish in small dishes, v\-ith microscopic examination of the infected parts 

 from fish killed during the time of infection and for several days following, or until it 

 is clear that the glochidia have become safely established in their host's tissues. After 

 even limited experience one learns approximately the number of glochidia needed and 

 can determine roughly their suspension in the water by taking samples at random in a 

 pipette, which when held against the light shows clearly the individual glochidia. Dur- 

 ing infection it is possible to pick out individual specimens and by hfting up the oper- 

 culum of the Uving fish, examine the gills with a hand lens. The glochidia are then 

 seen individually and the progress of the infection can be watched. Fin-infecting glo- 

 chidia may be seen individually if a fish is placed in a small dish against a black back- 

 ground. 



It is not difficult to determine by these means the optimum time for the exposure. 

 When 100 fish 5 to 6 inches in length are taken and the contents of a single marsupium 

 of a large Lampsilis is placed in an ordinary washtub, infections may be obtained some- 

 what as follows: Rock bass, exposed 30 to 40 minutes, 2,000 to 2,500 glochidia on gills 

 of each fish ; large-mouth black bass, exposed 1 5 to 20 minutes, 500 to i ,000 glochidia on 

 gills ; crappie, exposed 20 to 30 minutes, 200 to 400 glochidia on gills ; yellow perch, exposed 

 20 minutes, 400 to 600 on gills; German carp (with Anodonta), exposed 30 to 40 min- 

 utes, 200 to 500 on fins. These figures are given as starting points for anyone attempt- 

 ing artificial infections and can not be taken as representing the results of precise deter- 

 minations of optimum infections for the fish in question, because the means for deter- 

 mining the numbers and distribution of the glochidia have been only approximate. 

 It will probably always be necessary, in the practice of artificial infection on a large scale, 

 to have the fish examined microscopically by a properly trained observer, and this 

 will be particularly true in the beginning of this work in hatching establishments, 

 because the practical details of artificial infection on a large scale have yet to be solved. 



DURATION OF THE PARASITIC PERIOD. 



According to the experience of pre\'ious observers, the duration of the parasitic 

 period varies inversely with the temperature of the water (Scliierholz, 1888; Harms, 

 1 907- 1 909). Although we have found this to be true in general, our experiments have 

 not shown so definite a relation between temperature and parasitism as has been 

 described by Harms, for example, and it is quite possible that other factors, which are 

 obscure, exert a modifying influence upon the length of time the glochidia remain on 

 the fish. Harms found that the glochidia of /I worfowto completed the metamorphosis in 

 80 days at a temperature of 8° to 10° C; in 21 days at 16° to 18°; and in 12 days at 20°; 

 while in the case of the bookless glochidia of Unio (which are gill parasites) the period 

 was 26 to 28 days at a temperature of 16° to 17°. He is incUned to attribute the some- 

 what longer time required for the metamorphosis of Unio to the fact that the glochidia 

 in this genus when discharged are in a less advanced stage of development than are 

 those of Anodonta — a difference that exists between all bookless and hooked glochidia 



