624. BULLETIN OF THE BUREAU OF FISHERIES. 
After a time, however, they become quiet, and when removed from the presence 
of the glochidia behave normally, except that the more rapid respiratory move- 
ments may continue even for days afterwards. 
The fishes we have used in our experiments have shown marked differences 
in their ability to retain the glochidia both on the fins and on the gills, more 
especially on the latter. They vary even more strikingly with respect to their 
capacity for withstanding the injurious effects of gill infection, and further- 
more some fishes are undoubtedly much more susceptible than others, as the 
glochidia become attached, especially to the gills, more readily. 
Of the fishes which we have had under observation, the following species 
have proved to be the most satisfactory: rock bass, large and small mouthed 
black bass, green sunfish, and red-spotted sunfish. All of these species are very 
susceptible and readily infected with gill parasites, large numbers of which may 
be carried without apparent injury to the fish. A single individual of any of 
the above-mentioned fishes, not more than 3 or 4 inches in length, will success- 
fully carry at least 1,000 glochidia on its gills to the close of the parasitic period, 
a number that would speedily kill many other species. 
Yellow perch, blue-gill sunfish, and crappie, although quite susceptible, are 
far less resistent to the injury and quickly succumb to overinfection. Young 
bullheads are apparently very difficult to infect, and we have not succeeded 
in causing glochidia to become attached to either their gills or fins in appre- 
ciable numbers. 
German carp are easily infected on external parts with hooked glochidia, 
but thus far all efforts to infect this fish with gill parasites have been unsuccessful. 
The glochidia will attach themselves to the gill filaments, but for some unaccount- 
able reason implantation is imperfect and they soon drop off. Apparently the 
epidermis of this fish does not react to the stimulus in such a way as to lead to 
the formation of a permanent cyst. 
In still other cases it has been found that failure is due to the fact 
that, even after complete implantation on either the gills or fins, the walls of 
the cyst slough away in a day or two and liberate the glochidia. Whether this 
is caused by unfavorable physiological conditions in the fish or in the glochidia 
we are unable to say, but there are certain facts that would seem to indicate 
that such abortive implantations are the result of a physiological immaturity 
of the glochidia, which therefore fail to call forth the necessary response on the 
part of the fish’s tissues. 
DURATION OF THE PARASITIC PERIOD. 
The duration of the parasitic period is very variable. With respect to 
the glochidia of those species of mussels which remain gravid over winter the 
duration depends upon the time of year when the infection is made. Fishes 
