336 BULLETIN OP THE BUREAU OF FISHERIES. 



contend with many enemies.. They are then a part of the plankton and as such have to 

 contribute their share toward the support of all the varied life which feeds upon the 

 plankton. There are many animals which eat copepods and none of them are at all 

 particular as to the species. These free-swimming ergasilids are fully as toothsome as 

 other kinds and are as often eaten. The male never escapes this danger, but the female 

 does when she has once fastened to the gills of a fish. It sometimes happens, however, 

 that when the female is ready to fasten to a fish all the fishes suitable for hosts have left 

 the vicinity. Under such conditions the female parasite must die unless she can swim 

 far enough to find a host. 



The argulids swim about freely, even after reaching maturity, especially the males. 

 During this swimming they also become pact of the plankton and share in its dangers 

 and vicissitudes. Being external parasites, they are not compelled to find a particular 

 host, for they can remain temporarily upon almost any fish until their true host is found. 

 They are thus much less susceptible to the dangers of the plankton than the ergasilids, 

 and when they have once reached maturity they are thenceforth free from such dangers. 

 Their much larger size also operates in their favor, for they are too bulky to be caught 

 by most of the creatures which eat ordinary copepods. 



The lemaeopods have but a very short free-swimming period, a few hours at the 

 most, and during that time they, too, are subject to the dangers of the plankton. They 

 must not only survive these dangers but they must also find a particular host within 

 this brief period or they perish; and the same disaster often overtakes them that 

 happens to the ergasilids, namely, when they are ready to attach themselves there are 

 no suitable hosts available. 



The lemaeids also become a part of the free-swimming plankton at two separate 

 periods in their development. First during the nauplius and metanauplius stages, 

 when they are indistinguishable from all other copepods in the same stages, so far as the 

 dangers of the plankton are concerned. Then they spend the copepodid stages as 

 parasites upon the gills of some fish, apparently any that happens to be available. On 

 leaving this intermediate host they again enter the plankton and swim about freely 

 while a union of the sexes takes place. The male develops no farther, but the female 

 must seek a permanent host, and this time it must be a particular species of fish. 

 During this latter period, therefore, they are in the same condition as the lemaopods 

 and often experience the same trouble, namely, when they are fully developed there 

 are no suitable hosts available. 



It follows that the parasites are ordinarily held in check by these means, and if 

 they are to become anything of a menace to the fish there must be peculiar conditions 

 favorable to them and unfavorable to their hosts. The custom practiced by the bio- 

 logical station of seining the fish out of the "lakes" and slews that are likely to go dry 

 and putting them back into the main river is the best thing that could be done to get 

 rid of the parasites. We have just seen that the latter breed rapidly under the conditions 

 obtaining in the slew and that everything works together in their favor. By removing 

 the fish such breeding is at once stopped; all the parasite larvse and adults left in the 

 slew die, and the new conditions in the main river are such as to keep subsequent breed- 

 ing within due bounds. 



