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PROPAGATION OF FRESH-WATER MUSSELS. 625 
infected in the late fall and winter will carry the glochidia for from two and a 
half to nearly four months, while in spring, when the same kind of glochidia 
are used, the metamorphosis is completed and the young clams liberated in a 
little less than a month. This great difference in the length of time that the 
glochidia remain on the fish can not be due primarily to temperature, although 
this may have some influence, for we have observed the same differences in the 
laboratory, where the temperature of the water remains practically constant 
throughout the winter and spring. As the glochidia of the mussels which have 
the long period of gravidity are not normally discharged until spring and summer, 
it is quite possible that the fall and winter glochidia are physiologically immature, 
although capable of causing the formation of the cyst on the fish, and require 
for this reason a much longer time for the post-embryonic development. Struc- 
turally, however, they seem to be identical with those taken from the marsupium 
in the spring. 
In the case of the spring infections the young clams all leave the fish 
at about the same time, and within a very few days after the first are liberated 
the last to go have come off. ‘This is not true, however, of the fall and winter 
infections, for in these cases the setting free of the young mussels may be 
extended over a period of three or four weeks, or possibly more. 
We have no observations on the duration of the parasitic period in the case 
of the summer-breeding mussels. Although infections have been success- 
fully made with several of these species and kept under observation for ten or 
twelve days, we have not yet had an opportunity of following the metamorphosis 
through to the end. The period is, however, undoubtedly short, probably not 
more than four weeks. Harms (Zoologischer Anzeiger, bd. xxx1r, 1908) has 
recently recorded a parasitic period for two European species of Unio, a genus 
belonging to the group of summer breeders, of twenty-six to twenty-eight days. 
POST-LARVAL GROWTH OF MUSSELS. 
No data have been obtained on the rate of growth of fresh-water mussels 
beyond a short period after the metamorphosis, as we have not succeeded in 
keeping the young mussels alive in the laboratory for more than four weeks after 
leaving the fish. The amount of growth during that time was only a fraction 
of a millimeter. We have made, however, extensive plantings of young mus- 
sels which have been allowed to drop off the fishes in ponds and streams where 
the chances of reclaiming them in the future are very good, and it is hoped that 
by another year or two some data on the rate of growth will be available. Care- 
fully measured and weighed young mussels of different sizes have also been placed 
in wire cages and set in ponds and rivers, and it is possible that additional data 
will be obtained from these experiments. 
B. B. F. 1908—40 
