COPEPOD PARASITES AND MUSSEL GLOCHIDIA ON FRESH-WATER PISHES. 335 



sustenance. That amount may be small, but it is nevertheless a loss, and it weakens 

 the fish's vitality by just so much. The simple fact that a sufficient number of para- 

 sites can weaken or even kill a fish is enough to prove that each one does his share toward 

 that end and is therefore harmful. ,^nd here in the Mississippi Valley there are other 

 considerations which tend to greatly increase this influence of parasitism. 



2. The parasites, especially the ergasilids, are more numerous upon young fish; 

 one can scarcely examine a young crappie or calico bass 3 to 5 inches in length without 

 finding it infested with Ergasilus cseruleus, its particular parasite, and the same may be 

 said of the hosts of the other ergasilids. It is not quite as noticeable in the case of the 

 arguUds and lemseopods, although even here the smaller fish are the ones most frequently 

 infested. These young fish are like the yoimg of all animals, including even man. They 

 are growing rapidly; they need all the vital energy they can produce to carry on this 

 growth successfully, and hence they are more susceptible to the injurious effects of 

 parasitism than the matured adult. We thus find a maximum of numbers of parasites 

 at that very stage of development when there is a minimum of resistance on the part of 

 the host, and this greatly increases the influence of the former upon the latter. 



3. Again, the parasites are more numerous in the slews and cut-offs (so-called lakes) 

 than in the main river. This is due partly to the absence of a current, thereby enabUng 

 the parasite larva to swim about freely, and partly to the crowding together of the para- 

 sites and fish, which materially aids the former in their search for the latter; but in 

 these shut-off bodies of water the conditions are not as favorable to the fish as in the 

 open river, especially late in the season. There is not as much food, the water is not 

 as well aerated, and there is a keener struggle for existence. Furthermore, in these 

 slews the young fish far outnumber the older ones; these are the very places to which 

 they resort to escape their enemies. Scarcely a fish can be found in these "lakes" and 

 slews which is free from parasites, and towing reveals the presence of large numbers of 

 parasite larvae swimming about in search of a host. Thus the parasites attack their hosts 

 not only at the stage of development when they are most susceptible, but also in the 

 places and under the conditions when they are least able to withstand the attack, again 

 greatly augmenting the influence of parasitism. 



4. With the time, the place, and the conditions thus favorable to the parasites, 

 the latter respond quickly and show an abnormal increase in development. A far greater 

 number reach maturity than under less favorable conditions; these in turn breed, and 

 the number of larvae is increased a hundredfold; a considerable percentage find hosts, 

 thus crowding the gills of the young and already weakened fish. In this way parasites 

 that are comparatively harmless under ordinary conditions may, and often do, become 

 a serious menace to the life of the fish. 



These considerations are enough to show that the presence of even a few parasites 

 Is not a matter of indifference. Fortunately, under ordinary conditions the parasite 

 has an even harder struggle for existence than its host. In this struggle the different 

 kinds of parasites are affected differently, while the ultimate issue is the same for them all. 



The ergasilids swim about freely until they reach maturity. The male never be- 

 comes a parasite, but completes its life as a free swimmer, while the female seeks a par- 

 ticular host. During this comparatively long free-swimming period both sexes have to 



