152 MCXLLUSKS. 



and the shell, it will be covered with a layer of pearl. 

 This is the way that the pearls of commerce are produced ; 

 and inferior pearls are often found in dissecting our com- 

 mon river mussels. 



II. Reproduction. The eggs, as already noticed, are 

 fertilized, hatched, and the young are nurtured for a time 

 in the outer gills of the female. Their number is pro- 

 digiously great. The young, when found in the gills 

 (then called glochidicC), differ very markedly in appearance 

 from the adults, in their widely gaping, triangular valves, 

 hooked at the tip. When they are expelled from the gills 

 into the water, to shift for themselves, they fasten them- 

 selves by means of these hooks to some floating or swim- 

 ming object, preferably to the fins of a fish or to the tail 

 of a tadpole, where they lead for a time apparently a para- 

 sitic existence. Later they fall to the bottom, and begin 

 life independently. Doubtless multitudes of those that 

 fall to the bottom are buried in the shifting mud and 

 sand ; many more are eaten by other aquatic animals be- 

 fore their shells have attained sufficient size and strength 

 to afford them protection ; and even in adult life some are 

 eaten by minks and otters, and very many are left out on 

 the banks by receding floods to die of evaporation : so 

 that the vast number of eggs produced appears, in the 

 end, after taking all casualties into account, to be only 

 sufficient to maintain for the species its accustomed num- 

 bers, and to keep up the biological balance in the life of 

 the river beds. 



III. Voluntary Motion. The muscular system serves 

 the river mussel the double purpose of performing its 

 movements, and of holding it in its shell. The strongest 

 muscles are the adductors, which close the shell. Opposed 

 to these is only the hinge ligament, which, upon their 

 relaxation, opens the shell automatically. 



The foot is principally a muscular organ consisting of 



