342 BULLETIN OF THE BUREAU OF FISHERIES. 



like L. ligamentina or L. ventricosa. When artificially infected, each crappie will take 

 from 1,000 to 2,000 glochidia and sometimes even more. 



Turning now to the copepods, we find that while it carries on its gills only two species, 

 it is, nevertheless, the worst infected fish in the Mississippi River so far as numbers are 

 concerned. Hardly a crappie examined during the summer season failed to yield speci- 

 mens of one or both copepods, and frequently the number from a single fish reached into 

 the hundreds and sometimes came close to a thousand. The difference in size between 

 the glochidia and copepods make these numbers closely correspond, and the limit in 

 both instances is apparently determined only by the actual living space on the gills. 



The second fish on the list is the sheepshead, Aplodinotus grunniens, which serves as 

 a host for 1 1 species of mussels, and upon the gills of which the number of individual 

 glochidia is usually well up in the hundreds and frequently reaches into the thousands. " 

 This is an apparent exception to the rule, for while there is an external Argulus parasite 

 to correspond with the few fin glochidia, a careful examination of all the sheeps- 

 head gills that were available (about 500) failed to reveal a single copepod; but there 

 are certain facts which profoundly influence our judgment in the present instance. 



First, and of the greatest importance, this fish habitually feeds upon thin-shelled 

 mussels, crushing the shells with its powerful pharyngeal jaws. Whenever the shell of a 

 gravid mussel is crushed in this way the gills of the fish necessarily become infected with 

 the glochidia which are set free. L. loevissima and P. donaciformis are the ones whose 

 glochidia are found in greatest numbers, and these as well as most of the others have 

 papery shells. This method of infection is quite different from that in the crappie and 

 other fish and comes close to being artificial. Furthermore, such infection is practically 

 constant, in fact as constant as the feeding of the fish, and thus the gills are kept loaded 

 with glochidia all the time. The presence of these glochidia prohibits that of the cope- 

 pods, as will be shown later. The glochidia of the thick-shelled mussels like Q. heros 

 are obtained in the usual way and are much fewer in number. 



Again we find upon the sheepshead's gills, in addition to the mussel glochidia a 

 trematode ectoparasite, which exists in as great abundance as the copepods upon the 

 gills of the crappie. The presence of these worms may still further explain the absence 

 of copepods. 



After the sheepshead comes the sauger with six species of glochidia, the green sun- 

 fish with five, the bluegill and white bass with four each, and the gizzard shad, the large- 

 mouth black bass, the skipjack, and the calico bass with three each. Of these the 

 bluegill, the white bass, and the calico bass are each infested with the same two species 

 of Ergasilus as the crappie, in smaller numbers but still to a considerable degree. The 

 largemouth black bass carries a still smaller number of individual copepods but com- 

 pensates for it by being the host of five different species. The green sunfish, the gizzard 

 shad, and the skipjack have but a single copepod parasite on their gills, but they are 

 also really the host of but a single kind of glochidium, the others being found in such 

 small numbers that they can be regarded only as accidental infections. Having thus 

 determined that the same fish serve as hosts for both copepods and glochidia, a second 

 question naturally arises: 



** On the gill: ol one fish of this species 5,200 glochidia of /'. donarifprmis were found, and upon another fish 10,400 of the same 

 glochidia. 



