FRESH- WATER MUSSELS. 155 



known to feed directly upon mussels. It is certain, however, that the fresh-water mussel 

 beds harbor quantities of other small animal life, such as insect larvae, snails, and 

 worms, and are gardens for the food of fishes (p. 119); in this, probably, lies the prin- 

 cipal clue to the association of fish and mussels. 



Finally, an economic consideration should be emphasized. The conservation of the 

 fishes is as important to the preservation of the fresh-water mussel resources and the 

 industries dependent upon them as is the propagation and protection of mussels. The 

 disappearance, or the radical diminution in number, of certain species of fish would re- 

 sult in the complete or virtual disappearance of corresponding species of mussel. On 

 the other hand, if the growth of mussels in more or less dense beds produces conditions 

 which are favorable to the growth of fish food, and obser\'ations do so indicate, then 

 the disappearance of the fresh-water mussels would result in the diminution of the 

 food supply for fishes, and the conservation of mussels is important for the preser\'a- 

 tion of our resources in fish. 



PARASITISM AND IMMUNITY. 



It is worth while to inquire as to the effect of the glochidia upon fish. Are they 

 parasites in the setme sense as tapeworms or round worms? Do they sap the vitality of 

 the fish, and are they accordingly to be regarded as in the nature of a disease? While 

 the relation of the glochidium to the fish can not be fully stated in the present stage of 

 investigation, it can be said that the principal effect upon the'fish, at first, at least, is the 

 slight laceration of the gills caused by the attachment of the glochidium. The fish 

 quickly heals over this wound to inclose the glochidium and form a small cyst, and 

 after that there is in nearly all cases no evidence of further irritation or of material 

 detriment to the surrounding tissues, except as the cyst and glochidium are sloughed 

 off at the expiration of the proper period. 



The fish feels the attachment of the glochidia; it shows that b}' the flirting move- 

 ments which are made as infection begins, and it is known that excessive infections of 

 young fish, at least, may cause the gills to become so lacerated and inflamed as to pro- 

 duce the death of the fish (Lefevre and Curtis, 191 2, p. 165). The use of small fish is 

 avoided in experiments and operations conducted at Fairport, and as care is taken to 

 avoid excessive infections it can be said that of thousands of fish artificially infected 

 and kept under observation in experimental work at that place there has been no case 

 of death or evidently diminished vitality with evidence to implicate the glochidia as 

 cause. 



After the microscopic lesion of the gill is healed over, which usually occurs in the 

 course of a day, the commercial species of mussels generally make little demand upon 

 the fish. No doubt they derive some nourishment from the fish, but this must be very 

 slight, since the young mussels, after spending two or three weeks in undergoing meta- 

 morphosis, are found to be of the same size as before they attached to the fish." The 

 demands upon the energies of the fish caused by the glochidia are probably not greater 

 than those arising from a few extra movements. 



It has recently been learned that some fish acquire a certain immunity to glochidia, 

 thus being protected against too frequent repetition of infections. Reuling (191 9) has 



<* The mussels which grow in size while in parasitism (p. 149) are not commercial species. 



