REPRODUCTION IN THE UNIONIDiE 
109 
near the tips to one on the sides and thus the greater part of every 
filament was left unchanged and in full functional condition, 
while in other infections (large-mouthed black bass + L. liga- 
mentinus), where a much greater proportion of the glochidia were 
upon the sides of the filaments, the mortality of the fish was very 
heavy, although the amount of infection was much less. A gill 
of the latter fish, from a lot lightly infected with these glochidia 
is shown in fig. 13. The number estimated for this fish, which 
was four inches long, being only 450, is distinctly less than the 
optimum. 
Implantation upon the filaments occurs in a manner similar to 
that of the hooked glochidia upon the external parts, but much 
more rapidly. Figs. 15, 16, 17 and 18 show the appearance at 
15 minutes, 30 minutes, 1 hour and 3 hours after infection, and 
our observations showing that the cyst is completed within from 
two to four hours, agree with what Harms (^09) has found for 
gill infections. The proliferation will even continue after the 
gill has been cut from the fish and placed in a watch glass for 
observation under the microscope. An immediate result of the 
cyst formation is the obliteration of the lamellae upon either side 
of the gill-filament, which thus becomes smooth and slightly 
swollen in the vicinity of the glochidium. Figs. 13 and 19 show 
the general and detailed appearance of the cysts and the diver- 
sity in the angles at which the glochidia are attached. 
The older statement that the hooked glochidia are fin and the 
bookless gill parasites finds, therefore, confirmation from our work 
though it would be better to say that the hooked attach most 
successfully to large strong margins like those of the fins while 
the bookless to soft and fine filamentous structures like the gills 
in fish of moderate size. The reaction of bookless glochidia to 
blood, with respect to the part it plays in attachment, has 
already been described. 
Susceptibility of fish to infection 
The susceptibility of different fish to infection is a matter 
which we think has not been sufficiently considered by any previous 
