6 UNIVERSITY OF MISSOURI BULLETIN 



on gills and fins the same length of time, but, as before stated 

 the hooked glochidia often fall off the gills before they complete 

 the metamorphosis, while the hookless ones rarely ever remain 

 on the fins more than a few days. For this reason, gill and fin 

 infections must be studied separately. 



Harms (9) found that large fishes became infected more 

 heavily on the gills than on the fins, both by Unio and Anodonta 

 larvae, while small fishes were far more susceptible to fin-in- 

 fection. This result is not in accordance with my own experi- 

 ments, for I have found that infection of both gills and fins, 

 by hooked and hookless glochidia, occurs more heavily in the 

 case of large fishes than in that of smaller ones. Black bass 

 become infected much more rapidly than the sun-fishes and 

 crappie. In the case of the black bass, successful fin-infections 

 have not been obtained with the glochidia of Symphynota com- 

 planata, although this fish is very susceptible to gill-infections. 

 Occasionally, however, a few of these glochidia attach them- 

 selves to the fins, but they always drop off within a few hours. 

 Both the bass and sun-fishes will bear a heavy gill-infection. 

 Often more than one thousand glochidia become successfully 

 implanted on the gills of a fish not more than four inches in 

 length and complete their development there without injury 

 to the host. 



It has been generally held by previous observers that the 

 duration of the parasitism varies inversely with the tempera- 

 ture of the water in which the development takes place. For 

 example, Harms (8) found that the glochidia of Anodonta 

 completed the metamorphosis in 80 days at a temperature of 

 8-10 C, in 21 days at 16-18 C, and in 12 days at 20 C, 

 while in the case of Unio the length of the parasitic period at a 

 temperature of 16-17 C was 2628 days. He believes that 

 the longer period required for the metamorphosis of Unio is 

 due to their less advanced stage of development at the time of 

 the infection. Although my own experiments show a general 

 correspondence with Harms' results for both the hooked and 

 hookless types of glochidia, the relation between the duration 



