I50 



BULLETIN OF THE BUREAU OF FISHERIES. 



(text fig. 13), and, in such cases, the mussel must be generously nourished by the fish. 

 (See Coker and Surber, 191 1-) 



The duration of the parasitic period varies greatly with the season of the year during 

 which it occurs, and with other conditions which are not fully understood. The results 

 of some recent experiments indicate that glochidia of long-term breeders have a rela- 

 tively long infection period when they are infected upon fish shortly after maturing 

 and a relatively short period when infected after they have remained in the marsupial 

 pouches over winter; that is, young glochidia complete metamorphosis in parasitism 



more slowly than old glo- 

 chidia. The temperature of 

 the water seems to be one 

 of the factors governing the 

 duration of the parasitic 

 period, and doubtless the 

 vitality of the host fish is 

 another; but there is diver- 

 sity even among glochidia of the same species when infected on the same fish. Lefevre 

 and Curtis (1912, p. 168), for example, show under such circumstances variations from 

 9 to 13 days, and even from 13 to 24 days. The following instances (Table 17) from 

 records at the Fairport station are illustrative : 



Table 17. — Infections Showino Duration of PARAsmc Peeiod. 



Fig. 14.- 



-A dorsal view of a juvenile pink heel-splitter showing glochidial shell still 

 visible. (Xi8). 



" still carrying infection, Apr. 14. 1915. 



In about one week after attachment, as a rule, the wall of the cyst begins to assume a looser texttire, 

 the intercellular spaces becoming infiltrated with lymph, and from this time on to the end of the parasitic 

 period there is little further change in its structure. 



Before liberation of the young mussel, the valves open from time to time and the foot is extended. 

 By the movements of the latter the cyst is eventually rupttired, its walls gradually slough away, and the 

 mussel thus freed falls to the bottom (Lefevre and Curtis, 1912, p. 171). 



