FOOD AND PARASITES OF SOME FRESH-WATER FISHES. 



521 



Food. 



Insect larvfe 



Gamnmrids 



Snails, mostly Phy.sa ancillariii 

 Crayfish 



Fish in 

 which 

 found. 



Food. 



Plant remains 



Plankton 



Minnows 



Fish spawn... 



Fish in 

 which 

 found. 



The 39 perch in which insect larvas were found contained, as far as 

 we could determine, phryganid larvae in but 1 and dragon-fly larvas in 

 9 fish. Thirty-six of the entire number contained but a single kind of 

 food, 15 had 2 kinds, 2 had 3 kinds, and 3 were found with 4 kinds of 

 food. In nearl}^ every perch in which more than one kind of food was 

 present, insect larva? were found. Forl)es (h) gives the food of the 

 perch he examined and we note a great similarity to what we recorded. 

 He found that a number of fish were eaten by the perch he examined 

 from Lake Michigan, due, no doubt, to the smaller amount of insects, 

 Crustacea, and mollusks present in the large lake. 



The following table will show the kinds of parasites found and the 

 abundance and distribution of each kind in their hosts: 



Clinostomum helerostomum on gills, 8; copepod (En/asilus) on gills, 9; cestode and nematode liver 

 cysts, 39; cysts in mesentery, 6; cysts in wall of stomach, 2. 



The trematodes were nearly all Distomnni nodulosuin^ which was by 

 far the most prevalent parasite. The number of times trematodes 

 were found in the gall-bladder was far greater than given in the table, 

 mf&,ny fish having been examined without being recorded. In late 

 July and early August perch were found with a small immature 

 fluke present in considerable numbers in the gall-bladder, more than 

 half the specimens examined being so infected. It occurred to us that 

 this might be the young form of D. noduloswn^ which is pro1)able, 

 although, the specimens in the gall bladder being immature, it was 

 impossible to make a direct comparison. In more than half of the 

 perch examined the liver contained cysts; many of these we opened 

 and found that they inclosed either a young- cestode or a young nema- 

 tode. The nematode cj'sts were generally smaller and firmer than 

 those containing the cestodes, but it was impossible, unless eveiy one 

 was opened, to be sure of the contents. There was undoubtedly hxxt 

 a single species each of cestode and nematode forming the cysts. 



The 16 perch caught during February through the ice were taken 

 much farther from shore than any of the others we examined. Four- 

 teen of these had fed exclusivel}^ upon plankton. They contained as 

 large a proportion of encysted parasites as any of the other perch, 



