178 BULLETIN OF THE BUREAU OF FISHERIES. 



condition for earlv juvenile life in a wide range of forms. It is, moreover, interesting 

 to find in the Unionidse, as in many other lamellibranchs (e. g., Mya and Pecten) a 

 functional byssus in the early stages, though there is no such organ in the adult. 



As these results are very important and of convenience for reference in this paper 

 we may here quote Isely's conclusions in full. 



The facts noted above are closely related, not only to the ecology of the juvenile mussel, but also 

 to the ecology of the adult. 



1. They indicate the conditions essential for the most successful growth and early development 

 of the Unionidae. This kind of an environment gives a constant supply of oxygen and sufficient food; 

 is frequented by suitable fish; is free from shifting sand and silt accumulation. Those mussels that 

 drop from the fish in these favorable situations develop in large numbers, while the less fortunate, that 

 drop in shifting sand and silt, die early. 



2. In the study of the ecological factors that are inimical to mussel life more attention should be 

 given to the consideration of the juvenile habitat. Absence of gravel bars and stony situations may 

 sometimes explain the scarcity of the Unionidae in certain streams and lakes where frequently water 

 content has been thought the chief unfavorable factor. 



3. It is a well-known fact that in many streams certain stretches of mud bottom are found loaded 

 with mussels, while other areas, in the same stream, equally favorable from the standpoint of the habitat 

 of the adult mussels, have only scattering specimens.  



This distribution of the adults may be explained by the assumption (which is fairly well established 

 by experimental study and will be discussed in a later paper) that the average mussel seldom travels far 

 up or dovNTi the stream from the place where it begins successful development. Stretches favorable for 

 juvenile development thus come to be the centers of dispersal in the streams where they occur. As 

 a result, areas of mud bottom near these favorable habitats become loaded with mussels by migration. 



4. In the study of the life history of the Unionidae we may consider the embryonic, the glochidial, 

 the parasitic, the early juvenile, and the adult as distinct periods for separate and special study. 



These results of Isely's are clearly of very great importance in the problem of arti- 

 ficial propagation and it is to be hoped that his observations may be greatly extended 

 in the near future. The number of different species which he has found is a most 

 promising sign that he is on the right track, and we may hope that we shall soon reach 

 a satisfactory understanding of this stage of the life cycle hitherto so little known. 



At this point a word regarding the formation of beds may be opportune. It is a 

 familiar fact that many species are most likely to be found congregated in beds which 

 in some of the larger streams must have contained, before the shells came into commer- 

 cial use, numbers of mussels which are hardly conceivable. Elsewhere in the stream 

 the mussels are found scattered and wandering over the bottom. In the absence of any 

 indication that the individuals of a species are in some manner attracted to one another, 

 the simplest explanation of the formation of beds would be the same as that given in 

 other cases of this sort. The conditions of food supply, current, character of bottom, 

 etc., must differ considerably, and we may reasonably suppose that some places present 

 the optimum conditions over an extended area and that in such a place a bed may be 

 formed. As the mussels wander over the bottom they may by chance enter such an 

 area of optimum conditions and will then move about less actively or come to rest, 

 because in the absence of unfavorable conditions there is no stimulus to continued loco- 

 motion. The result is that individuals which enter are likely to remain and more keep 



