346 BULLETIN OF THE BUREAU OF FISHERIES. 



vol. XXVIII, pt. I, p. 622). Here again the mere presence of a few copepods upon the 

 gills of a fish could have no effect upon such a stimulus. The respiratory movements of 

 the fish may have considerable to do with it; the crappie's respiration is not very vigor- 

 ous even at its best, and this is especially true of small fish (Lefevre and Curtis, Journal 

 Experimental Zoology, vol. 9, p. 103). 



The irritation due to the presence of parasitic copepods may still further reduce 

 these movements and thus prevent infection by glochidia ; but if this were the only cause 

 ID copepods could hardly produce so large an effect. It would seem as if there must be 

 something further, either chemical or physiological in its action, in order to accomplish 

 the known results. It will not be very easy to prove what this is, but meanwhile the 

 facts remain unaltered that in some way the presence of a very few copepods greatly 

 reduces the fish's susceptibility to infection by glochidia. 



2. As the number of copepods upon a fish's gills increases its susceptibility to 

 infection by glochidia diminishes. Naturally a limit is soon reached beyond which the 

 susceptibility has diminished so much that practically there can be no infection at all; 

 this limit for small crappies is about 200 copepods. If more than this number is present, 

 the glochidia are very scattering and are usually below 50 in number. The copepods 

 often increase to 500, and in such instances there are no glochidia, or, if any, their 

 number is expressed by a single digit. 



Certain conclusions naturally follow from these facts. The first is that it is obvi- 

 ously disadvantageous to attempt to infect with glochidia fish that are already carrying 

 copepods. A few glochidia will always stick to their gills, but not in sufficient numbers 

 to repay the labor expended. Since the large fish are relatively freer from copepods 

 than the smaller ones, it follows that they make the better hosts. Not only are their 

 gills larger and thus capable of carrying more glochidia, but the latter will fasten to 

 them more readily because of the comparative absence of copepods. 



Again, the fish from the main river, whatever their size, make better hosts than those 

 from the slews and "lakes," because they, too, are freer from copepods. This is espe- 

 cially true at those times when the water is very low ; during a long-continued drought 

 it would be of little use to try infecting fish caught in such places because they would 

 be so infested with other parasites that very few of the glochidia would fasten to them. 

 The best thing to do with such fish would be to replace them in the main river and trust 

 to taking them again after they had gotten rid of their copepods. 



3. It is obviously a poor rule that does not work both ways, and we find that the 

 presence of glochidia is as prohibitive to the copepods as are the latter to the former. 

 This also is something that can not be watched under natural conditions; neither can 

 it be proved by experiment, for we can not supply parasitic copepods as freely as we can 

 glochidia; but it is abundantly sustained by a study of natural infections on the gills of 

 fish taken in the river. There are in the possession of the biological station about i ,000 

 vials of gills showing natural infection by various glochidia. These were all carefully 

 examined for parasitic copepods under a dissecting microscope, and in not a single 

 instance where the number of glochidia exceeded 300 was there even a single copepod 

 present. 



This mutual antagonism between the copepods and glochidia enables us to under- 

 stand clearly why the sheepshead's gills are never infested with copepods. From the 

 nature of the fish's food, as already explained, its gills are kept crowded with glochidia 



