REPRODUCTION AND ARTIFICIAL PROPAGATION OF FRESH-WATER MUSSELS. I 77 



pi. xii) and then ceased growing although they continued to move actively about. 

 This would indicate that the difficulty lies in the lack of a suitable food supply. Crus- 

 tacea were not obser\'ed to play an important role, though we do not doubt the cor- 

 rectness of Harms's observations in this respect. 



Figures 18, plate viii, 47 and 48, plate xii, will illustrate the appearance of the 

 young mussels at this period and an examination of figure 47 will show how extensively 

 the organs of the future adult have been laid down. Nothing remains to suggest the 

 glochidium save the shell, and structure and habit alike indicate that the organism is 

 now ready for a life on the bottom essentially like that of the adult. 



JUVENILE STAGES AND THE ORIGIN OF MUSSEL BEDS. 



For the sake of completeness, we shall discuss briefly at this point the present state 

 of our knowledge regarding the stages between the one last mentioned and that repre- 

 sented by the young mussels over 20 mm. in length, which are often found upon the 

 natural beds. In common with the experience of other collectors, we have seldom 

 found mussels under 20 mm. It would therefore seem clear that these early stages 

 are not at all common in localities where the slightly later stages and the adults are 

 found. Isely (1911) has published a preliminary note upon his study of this "juvenile" 

 period. We shall refer to his results rather fully, since there are no other recorded 

 observations which deal with these stages save in the way of incidental reference to 

 single specimens. This author states the problem by saying (p. 77) that: "Much diffi- 

 culty was experienced in finding young mussels for study and experimentation. I have 

 collected many specimens from the size of a nickel (20 mm.) to a quarter (24 mm.), but 

 mussels under the size of a dime (17 mm.) have been rare." The latter he terms the 

 "early juvenile" stages, including in this "the period following the time when the 

 mussel completes the parasitic stage and leaves the fish to lead an independent life 

 until it is about 15 mm. in length. This would cover, in most species, approximately 

 the first year of independent existence. Other periods may be designated as later 

 juvenile and adult life." He then reports the finding of 32 specimens in this early 

 juvenile stage representing four genera and nine species, as follows: (i) Lampsilis 

 luteola, two; (2) Lampsilis jallaciosa, one; (3) Lampsilis parva, four; (4) Lampsilis 

 gracilis, three; (5) Plagiola elegans, one; (6) Plagiola donacijormis , sixteen; (7) Anodonta 

 imhccillis, two; (8) Ptychobranchus phaseolus, two; (9) unnamed species, one. 



All these specimens were found in places where the water was fairly swift, from 

 I to 2 feet in depth, and on a bottom of coarse gravel, the particles of which were 10 

 to 25 mm. in diameter. They were anchored by the threads of a byssus gland "strong 

 enough to support the mussel in a rapid current" and capable of sustaining "the weight 

 of a number of small pebbles without breaking." 



Here then, as Isely concludes, we have the clue to the habits and ecology of these 

 so little-known stages. The finding of representatives from so many genera and species, 

 both heavy and light shelled, under identical environmental conditions and the presence 

 of the functional byssus in all cases is pretty good evidence that this is the normal 



