622 BULLETIN OF THE BUREAU OF FISHERIES, 
however, do not as a rule acquire a permanent lodgment on the delicate gill 
filaments, as their hooks usually tear through these when grasping. Their 
greater size would also seem to render it difficult for the hooked glochidia to 
become properly embedded under the epidermis of the slender filament. 
Hookless glochidia, on the other hand, are only rarely found on the external 
parts of the fish, and although they become attached readily to such places, in 
the absence of hooks their grasp is insecure, and the usual consequence is that 
they are all soon brushed off. Being hookless and quite small, they are admi- 
rably adapted to lodgment on the gill filaments, among which they may become 
entangled in great numbers. 
Interesting differences of a physiological nature also are correlated with 
the two structural types, apparently in adaptation to their different parasitic 
habits. 
IMPLANTATION OF THE GLOCHIDIA. 
Contact with any part of the fish affords the stimulus which causes the 
adductor muscle of the glochidium to contract and close the valves. This stim- 
ulus is purely a mechanical one, as the same result may be produced by touch- 
ing with any object. In this way they may be caused to snap shut on a bristle 
or hair, or on the edge of a piece of paper. Once attached to the fish, how- 
ever, the glochidium exerts a stimulus upon the epidermal cells of the host 
which causes them to undergo a rapid proliferation. The stimulus in this case 
is undoubtedly a chemical one, and its immediate effect is to throw the cells 
of the epidermis into active mitotic division. The epidermis now grows rapidly 
over the glochidium, which may become completely embedded in three hours 
after attachment in the case of the gill parasites. The proliferation of the epi- 
dermis of the fins occurs more slowly and in this situation twelve to twenty- 
four hours are usually required before the glochidia are covered. 
After the process of implantation is completed, the glochidia are entirely 
inclosed within the cellular cyst, where they are nourished by the host until 
liberated by the rupture of the cyst at the end of the parasitic period. It is 
during this stage of the development that the metamorphosis occurs and the 
structure of the glochidium is replaced by that of the mussel. 
EXPERIMENTAL INFECTIONS. 
METHOD. 
A very large number of experimental infections have been made, and the 
conditions determining infection have been investigated as far as the material 
at our disposal has allowed. This study has included not only the behavior of 
both types of glochidia, their manner and place of attachment, but the reac- 
tions of the fish as well. 
