COPEPOD PARASITES AND MUSSEIv GI^OCHIDIA ON FRESH-WATER FISHES. 345 



Nothing of this sort can be tried with the copepods, since we can not supply larvae 

 in the right stages of development as we can glochidia. But although our knowledge of 

 both kinds of parasites is rather limited as yet, enough data have been accumulated to 

 show that the two kinds of parasites behave very similarly in regard to their hosts. 

 There is thus a decided similarity between them when each is found b}' itself upon some 

 suitable host. 



III. Does the actual presence of copepods on a fish's gills exert any influence upon its 

 susceptibility to infection by glochidia? 



In other words, granting that the same fish do serve as hosts for both glochidia and 

 copepods, are the conditions favorable for both at the same time? This is manifestly 

 something which can not be watched under natural conditions, and the only way to 

 answer the question is by artificial infection experiments. Accordingly a hundred crap- 

 pies, Pomoxis annularis, of nearly uniform size (5 to 6 inches long), which had been 

 caught and brought to the station for artificial infection, were carefully examined and 

 25 we^re found to be infested with Ergasilus cceruleus, while the other 75 were free from 

 them. The entire hundred were then infected in the usual manner and under exactly 

 the same conditions with the glochidia of the black sand-shell, Lampsilis recta. After 

 infection the 25 parasitized fish were killed, their gills were removed, and the number 

 of copepods and glochidia on each was counted with the following results : 



The average number of glochidia upon each of the nonparasitized fish was between 

 1 ,000 and 1 ,200. By comparing this with the numbers given in the table we deduce the 

 following : 



I . The presence of even a small number of copepods upon the gills of a fish reduces 

 its susceptibility to infection by glochidia to one-third or one-fourth of what it would 

 be if no copepods were present. 



Even the gills that contained 10 copepods or less showed the presence of only a 

 few hundred glochidia instead of the thousand or more upon a nonparasitized fish. 

 Such a marked reduction can not be explained by the mere presence of the copepods; 

 they do not occupy enough of the gills to exert any crowding influence, neither are they 

 ever found attached to the tips of the filaments where the glochidia mostly congregate. 

 Manifestly there is room enough for both kinds of parasites without serious crowding; 

 gills that will accommodate 1,200 glochidia with no apparent injury to the fish can 

 certainly find room for more than 400 when only 10 copepods are present. 



Lefevre and Curtis say that the stimulus which causes the glochidium to close and 

 thus to fasten itself to the fish is purely a mechanical one (Bulletin Bureau of Fisheries, 



