l62 BULLETIN OF THE BUREAU OF FISHERIES. 



In a fish which will carry a given glochidium successfully, over-infection of the 

 gills is easily accomplished and easily fatal, although species of fish differ greatly in the 

 amount of infection they are able to withstand without serious mortality. In one 

 of our most successful combinations (rock bass infected with Lampsilis ligamentina) , 

 fish 4 inches in length were estimated to be carrying in the neighborhood of 2,500 

 glochidia, an average of more than two for every filament of the gills and yet there was 

 almost no mortality among the fish. A rock bass from this infection is shown in text 

 figure 2, which also illustrates the distribution of the glochidia on a single fish. lu 

 this case the success of so heavj^ 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 near the tips to one on the proximal two-thirds of the filament, and thus the greater 

 part of every filament was left unchanged and in full functional condition, while in 

 other infections (large-mouth black bass with L. ligamentina), where a much greater 

 proportion of the glochidia were upon the sides of the filaments, the mortality of the 

 fish was heavy, although the amount of infection was much less. A gill of the latter 

 fish infected with these glochidia is shown in figure 39, plate xi. The number estimated 

 for this fish, which was 4 inches in length, being only 450, is 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. Figures 35, 36, 37, and 38, 

 plate XI, show the appearance at 15 minutes, 3ominutes, i hour, and 3 hours, respectively, 

 after infection, and our observations, showing that the cyst is completed within from 2 

 to 4 hours, agree with what Harms (1909) has found for gUl infections. The prolifera- 

 tion wUl even continue af tet the gill has been cut from the fish and placed in a watch glass 

 for observation under the microscope (fig. 54 and 55, pi. xiii). An immediate result of 

 the cyst formation is the obliteration of the lamellae upon either side of the gUl filament, 

 which thus becomes smooth and slightly swollen in the vicinity of the glochidium (fig. 43, 

 pi. xi). Figures 34 and 43, plate xi, show the general and detailed appearance of the 

 cysts and the diversity in the angles at which the glochidia are attached. 



The older statement that the hooked glochidia are fin and the bookless gUl parasites 

 finds, therefore, confirmation from our work, although it would be better to say that the 

 hooked attach most successfully to large strong margins like those of the fins, and the 

 bookless to soft and fine filamentous structures like the gills in fish of moderate size. 

 The reactions of the two types of glochidia to mechanical and chemical stimuli, with 

 respect to the part they play in attachment, have already been discussed. 



SUSCEPTIBILITY OF FISHES TO INFECTION. 



The susceptibility of different fishes to infection is a matter which has not been suffi- 

 ciently considered by any previous investigators. We have evidence that some species 

 are much less susceptible than others to one or the other type of glochidium, and that in 

 these cases any considerable infection is an impossibility. The most striking instances 

 of this are the German carp, certain minnows, and the darters. 



