PROPAGATION OF FRESH-WATER MUSSELS. 623 
Twenty different species of mussels, yielding both hooked and hookless 
glochidia, have been used for the infections. The majority of these have been 
species showing the long period of gravidity, although several of the summer 
breeding species have also been employed. Eighteen species of fishes have 
been used, and their reactions have been carefully observed, but, as all are 
not equally favorable for receiving and carrying the parasites, the greater num- 
ber of infections have been made upon five or six species. 
When only a few fishes are to be used at a time, the method which has been 
employed is quite simple. The brood chambers are excised from the gravid 
mussels, split open lengthwise, and the ripe glochidia washed and scraped out 
into dishes of water. ‘The fishes are then placed together with the glochidia in 
small aquaria, and, if the infection is to be made with gill parasites, the water 
is agitated by mechanical means, in order to keep the glochidia suspended and 
thus insure their being taken into the mouth of the fish. When using hooked 
glochidia for external attachment, however, it is better not to stir them up, as 
the fishes usually settle down to the bottom of the vessel and, by rubbing their 
fins among the glochidia, quickly become infected to any desired degree. 
When making gill infections, the fishes must be examined at intervals of 
one or two minutes for the purpose of observing the rapidity with which the 
glochidia are attaching, and when the degree is reached which experience has 
* shown to be the optimum for the species of fish in question, the exposure is at 
once discontinued. Usually the gill infections take place very quickly, and 
in four or five minutes the fishes will have received as many: glochidia as they 
can safely carry, although of course the duration of exposure depends upon 
the abundance of glochidia in the water. Other things being equal, the fewer 
glochidia present the more prolonged the exposure must be. Experience has 
shown that the best results are obtained when the glochidia are used in very 
‘‘dilute mixtures,” for it is difficult to avoid overinfection, if they are at all 
concentrated. Some species of fishes, however, are less susceptible than others, 
and a longer exposure is necessary to insure the attachment of an adequate 
number of glochidia. In certain cases we have had to allow the fishes to 
remain with the glochidia for from two to three hours, or even longer, in order 
to obtain a satisfactory degree of infection. 
In nearly all of our experiments we have used young fishes, from three to 
six inches in length, as for many reasons they are more convenient than larger 
ones. 
BEHAVIOR OF FISHES—THEIR SUSCEPTIBILITY AND RESISTANCE. 
The fishes when first placed in contact with glochidia give evidence of great 
irritation, and as soon as the gills begin to be infected they exhibit violent and 
rapid breathing movements, apparently in the attempt to expel the parasites. 
