288 Adjustment to Conditions of Aquatic Life 



Fig. 176. A gravid mussel (Symphynota 

 complanata) with left valve of shell and 

 mantle removed, showing brood pouch 

 (modified gill) at B. (After Lefevre and 

 Curtis.) 



of livelihood during early development the young 

 of mussels have mostly taken on parasitic habitSc 

 They attach themselves to the fins and gills of fishes 

 (fig. 175). There they 

 feed and grow for a 

 season, and there they 

 undergo a metamor- 

 phosis to the adult 

 form. Then they fall 

 to the pond bottom 

 and thereafter lead 

 independent lives. 



The eggs of the river- 

 mussels are passed in- 

 to the watertubes of 

 the gills where they 

 are incubated for a 

 time. Packed into these passageways in enormous 

 nimibers they distend them like cushions, filling them 

 out in various parts of one or both gills according to 

 the species, but mostly filling the outer gill. When 

 one picks up a gravid mussel from the river bed the 

 difference between the thin normal gill and the gill that 

 is serving as a brood chamber (fig. 176) is very marked. 



GlocJiidia — In the case of a very few river mussels 

 {Anodonta imbecillis, etc.) development to the adult 

 form occurs within the brood chamber; but in most 

 river mussels the eggs develop there into a larval form 

 that is known as a glochidium. This is already a bivalve 

 (fig. 177) possessing but a single adductor muscle for 

 closing the valves and lacking the well developed system 

 of nutritive organs of the adult. It is very sensitive 

 to contact on the ventral surface. In this condition 

 it is cast forth from the brood chamber. 



