\^ REPRODUCTION IN THE UNIONID^ 105 
of the glochidia becone attached to the fin margins. If a fish 
is carefully watched, as its slight movements stir up the glochidia 
during the infection, the latter are seen continually falling upon 
the upper faces of the pectoral and pelvic fins. They may even 
be collected with a pipette and heaped upon a motionless pectoral, 
remaining there for some minutes without more than an occa- 
sional specim.en becoming attached. The margin of the fin is so 
much more favorable for attachment, that it is oiten thickly set 
with glochidia, when none are found upon the fin surface, and 
this despite the fact that glochidia must, during infection, strike 
against the surfaces of the fins, many times for every time that 
one of them comes in contact with a fin margin. It is, therefore, 
the fin margin for which this glochidium is best suited, and once 
fastened there, it is almost certain to remain and become over- 
grown. When a specimen does fasten to the surface, it probably 
gains its hold by catching upon one of the ridges formed by the 
fin rays, for the hooks could hardly be used upon a perfectly flat 
surface. Glochidia sometimes hold to the surface of a fin by a 
shred of tissue, under which their hooks have caught and remain 
there after all their fellows (fig. 11) are completely overgrown, 
only to be torn off later without having caused any noticeable 
hypertrophy of the fin tissue. Figs. 11 and 14b show that glochi- 
dia may become overgrown either flat against the surface, or upon 
edge, and fig. 12 shows a young mussel leaving a surface attach- 
ment after a parasitism of seventy-four days. 
The distribution of the glochidia to the several fins is deter- 
mined solely by the number likely to be brought in contact with a 
given part of the body. Those fins which brush against the 
bottom are always the more heavily loaded and the numbers 
elsewhere depend upon the extent to which the glochidia are kept 
suspended on the water. 
Optimum infections, as we shall term those which are close 
upon the limit of the number of glochidia which a fish can safely 
bring through the metamorphosis, often show the glochidia very 
closely set one after another, as in fig. 11, and several hundred 
may be safely carried by a fish three or four inches in length. 
Prolonged exposure causes so heavy an infection of the margins 
