THE LIFE OF THE UNIO 57 



until they die unless they are fortunate enough to become 

 attached to a fish on which they pass through post-em- 

 bryonic development as parasites. Multitudes of glochi- 

 dia die by not being attached to the fishes. In most cases 

 they attach themselves to the gills of fishes, although 

 they also attach to the fins and tails of fishes. There are 

 several types of glochidia. One is the hooked type 

 which is provided with tiny hooks, which clasp the fila- 

 ments of the fishes' gills and some times attach to the 

 fins or tail. 



Another kind is the hookless type and the tiny mussels 

 cause the sharp edges of the shells to clasp the filaments 

 of the gills and the edges of the fins and tail in about 

 the same manner that the hooked type becomes attached. 



There is also another type called the Proptera or ax- 

 head type. The shape resembles that of an ax-head. 

 This glochidium possesses hooks, but differs from the 

 other hooked variety. It has four hooks which close to- 

 ward each other and hold tightly wherever they attach. 

 They seem to prefer to attach to the filaments of the 

 fishes gills. A week after the attachment, the wall of the 

 cyst becomes somewhat looser and later the valves open 

 occasionally and the foot is extended and finally breaks 

 out of the cyst and the free mussel falls to the bottom. 



It is said that if these tiny mussels fall in muddy 

 places they perish, while if they fall in sand and gravel 

 they thrive and form colonies or beds. 



Some of the fishes are much more susceptible to in- 

 fection than others. The rock bass is especially suscepti- 

 ble, and according to the U. S. Bureau of Fisheries, 2,000 



