UNIVERSITY OF MISSOURI BULLETIN 



supium over winter is merely an adaptation for protection 

 against cold. 



Unio complanatus, on the other hand, is one of the so- 

 called "summer-breeders," in which the embryonic develop- 

 ment occurs in late spring and summer months and the glochi- 

 dia are discharged as soon as they are fully formed. 



There are two distinct types of glochidia, the hooked and 

 the hookless. The glochidia of Symphynota complanata are of 

 the former type, having at the apex of each valve a strong hook 

 provided with numerous teeth on its outer surface. The glochi- 

 dia of the other three forms studied, Lampsilis ligamentina, L. 

 recta, and Unio complanatus are entirely hookless. As is usually 

 the case with hooked glochidia, those of Symphynota complanata 

 are much larger than the hookless ones and differ from them in 

 shape. For example, the glochidia of the former measure 0.30 

 x 0.29 mm., while those of Lampsilis ligamentina and L. recta 

 measure only 0.24 x 0.20 mm., and those of Unio complanatus 

 0.20 x 0.21 mm. 



Hooked glochidia are best adapted for attachment to 

 external parts of the fish, as the margin of the fins, fin-rays, 

 operculum and mouth, but they are often found also on the 

 gill-arches and rakers. Large numbers of glochidia of Symphy- 

 nota complanata do quite often grasp the gills, but they are 

 apt to tear entirely through the delicate filaments with their 

 stout hooks, and even if they do succeed in becoming properly 

 attached, their large size seems to make it somewhat difficult 

 for them to become covered by the epithelium of the gill- 

 filaments. If once completely embedded, they may remain in 

 this condition until the end of the metamorphosis, but in many 

 cases the wall of the cyst is apparently not strong enough to 

 retain them on the filament, the cyst either rupturing and allow- 

 ing the glochidium to escape or breaking off below the point 

 of attachment of the glochidium which, in this event, falls to 

 the bottom still surrounded by the epithelial cells. After such 

 premature liberation from the fish, no further development 

 occurs, and the glochidium soon dies. 



