170 BULLETIN OF THE BUREAU OF FISHERIES. 



seine along with other fish and were infected the same as the rest. Of the 150 specimens 

 examined 53 had been thus artificially infected with glochidia, a trifle more than a third 

 of the whole, and upon these not a single copepodid larva was found. 



In many instances both infected and uninfected gills were removed and placed 

 together in a jar of water over night. The gills that had no glochidia were well covered 

 with copepodid larvae and many specimens were obtained from them on the following 

 morning. But none of these larvae would fasten on the gills that were already occupied 

 by glochidia. 



In wandering about the aquarium the larvae necessarily came in contact with both 

 kinds of gills, but would not fasten on those that had glochidia. Hence there must be 

 something in connection with the glochidium-infected gill that becomes manifest to the 

 copepodid larva and exerts upon it an antagonistic influence. What this is it would 

 be very difficult to decide, but it would seem as if it must influence the larva through 

 sensations corresponding either to smell or taste. 



And the antagonism is not all in one direction; the presence of copepodid larvae 

 proves fully as distasteful to the glochidia as does that of the glochidia to the larvae. 

 On the gills above mentioned that were well infested with copepodid larvae no glochidia 

 were found, or if present they were in such small numbers (one to four) as to prove the 

 general statement still more forcibly. Some of these fish must have had the same 

 chance as the others to become artificially infected with glochidia; the very fact that 

 they were ca!ught and brought in for examination proved this. 



Finally to complete the evidence about a dozen fish were found with a respectable 

 infection of both glochidia and copepodid larvae, about half a dozen of each. This would 

 seem to indicate that the antagonizing influence, whatever it may be, can not be spread 

 over the entire gill by a few parasites of either kind, but that it requires a dozen or more. 



When the smaller number is present the antagonism is incomplete and both kinds 

 may be found upon the same gill, but when the larger number is present the antagonism 

 becomes complete and one kind of parasite occupies the gill to the exclusion of the other. 



Two conclusions naturally follow from these observations, the one being the reverse 

 of the other: 



1. If conditions occur which are peculiarly favorable to thecopepods and they in- 

 crease so as to be present in considerable numbers on the gills of all the fish, then artificial 

 infection with mussel glochidia will be seriously hindered and may even be a failure. 



2. But evidently this artificial infection with glochidia and any natural infection 

 that may occur will operate as a preventative against the increase of the copepod para- 

 sites, and if persisted in might reduce their numbers almost to extinction. At all events 

 among fishes kept for the purpose of breeding mussel glochidia there will be very little 

 danger of such an epidemic of copepod parasites as often occurs in trout ponds and 

 aquaria, due to excessive development. 



Geographical distribution of species. — Of the 17 species here enumerated 9 (53 per 

 cent) are from our North American rivers and lakes, 4 (24 per cent) are from Africa, 3 

 (17 per cent) are from Europe, and a single one (6 per cent) is from South America. 

 The following list gives the names of the species found on the respective continents : 



North America: anomala, n. sp., catostomi, cruciaia, dolabrodes , n. sp., pectoralis, pomoiidis, tenuis, 

 toriua, variabilis. 



Africa: barnimii, diceracephala, haplocephala, temnocephala. 

 Eunjpe: cyprinacea, esocina, phoxinacea. 

 South America: lagenula. 



