108 GEORGE LEFEVRE AND WINTERTON C. CURTIS 
become attached to and even embedded upon the fins and other 
external parts. Harms ('08) concludes that the hookless type 
persists in much greater numbers on the fins of small than of large 
fish and that the hooked type will survive upon the gills if large 
enough fish are used, and it is doubtless true that the size of the 
gills and fins is an important factor in determining the place of 
attachment for each type, since the hookless form is only adapted 
for holding to a delicate surface like the gill-filament, or a fine fin 
while the hooked seem likely to be easily torn from just such a 
surface. When the hookless form does once become established 
upon an external part, it will develop there without mishap as 
shown by a figure of a hooked and hookless glochidium develop- 
ing side by side upon the margin of a fin (fig. 23). Within the 
mouth cavity, the glochidia will attach to the gill-bars and rakers 
if these parts are covered by a sufficiently delicate epithelium, 
but upon the gill-filaments they are always found in the greatest 
numbers. Tn most of our infections the filaments are the more 
heavily infected toward their outer ends (fig. 19), but the dis- 
tribution varies somewhat with the species of fish. For example, 
successful infections of rock bass with Lampsilis ligamentinus 
show about seven glochidia upon the distal third of the filament to 
one upon the proximal two-thirds; of large-mouthed black bass 
about three to one; and of yellow-perch about one and a half to 
one; differences which are probably caused by some particular 
configuration of the mouth cavity which causes the glochidia to 
fall more upon one part of the filam.ents than another. 
In a fish which will carry a given glochidium successfully, over- 
infection of the gills is easily accomplished and easily fatal, but 
the species of fish differ greatly in the amount of infection they 
are able to carry without serious m.ortality. In one of our most 
successful combinations (rock bass + Lampsilis ligamentinus). 
fish four inches in length were estimated to be carrying in the 
neighborhood of 2500 glochidia, an average of more than two for 
every filament of the gills and yet there was almost no mortality 
am.ong the fish. In this case the success of so heavy an infection 
is perhaps explained by the distribution of the glochidia upon the 
gill-filaments, for we found by count that there were about seven 
