194 BULLETIN OF THE BUREAU OF FISHERIES. 



presence of parasites, the possibility of producing them at will offers an interesting 

 opportunity for experimental study. 



The Unionidse, in short, are a group of animals which, for the great variety of 

 problems, both scientific and economic, presented in their unique life history, their 

 structure, functions, and habits, their many interesting adaptations, and in their 

 economic relations, is scarcely excelled by any other invertebrates except the insects. 

 At present we may be said to possess only an introduction to a knowledge of the family, 

 and the writers of this paper will feel amply repaid for their labor if they have succeeded 

 in exposing some of the problems which here lie open for investigation and at the same 

 time in laying the foundation for the artificial culture of fresh-water mussels. 



