152 



BULLETIN OF THE BUREAU OF FISHERIES. 



The experimental method is simpler in some respects. It consists in submitting 

 various species of fish to infection with the glochidia of a given species of mussel and 

 observing whether or not the glochidia attach. Since glochidia will sometimes attach to 

 fish which are not their natural hosts, it is necessary to hold the fish under observation 

 until the mussels have completed the metamorphosis and dropped off. It is, however, 

 impracticable to have on hand all the species of fish at the particular time when the 

 glochidia of a given species of mussel may be available. Furthermore, the failure of an 

 artificial infection to go through successfully on fish held in confinement may be due, 

 not to the want of a natural affinity between mussel and fish, but to the fact that the 

 fish does not retain its full vitality in close confinement, or to some other defect in the 

 experimental conditions. Neither of the two methods for the study of infections may, 

 then, be relied upon exclusively for the determination of the natural hosts of fresh- water 

 mussels. On the contrary, it has been found necessary to carry on the two lines of study 

 hand in hand, according to the plan which was adopted at the beginning of the scientific 

 work of the station. In this way, though our knowledge of the hosts of mussels is as 

 yet incomplete, there has been obtained a considerable body of information most of 

 which is summarized in the following table (i8),° listing 17 species of mussel and 30 

 hosts (29 fishes and i amphibian), and indicating those which serve as hosts for each 

 species of mussel. 



EXPLANATION OF TABLE i8. 



N. Found on the gills in natural infection. 



Nf. Found on the fins in natural infection. 



n. Record of natural infection but of doubtful significance. 



A. Carried through on gills after artificial infection. 



Af. Carried through on fins after artificial infection. 



a. Results of artificial infection unsatisfactory or not uniform. 



o. Tested and found unsuitable. 



T. Tested; development occurred; host perhaps suitable, but experiment not carried to conclusion. 



Table 18. — Commercial Mussels and Their Hosts. 



^ A great many data regarding the hosts of noncommercial species of mussels had been accumulated, but unfortunately most 

 of the records applying to such species were destroyed with the burning of the laboratory in December, 191 7. 



