353 



attacks of boring' sponges and cai-nivoroiis gastropods. 

 When the animals die, theii- valves commonly fall apart, and 

 from their position, and the character and direction of the 

 waves and currents, one valve may be carried shoreward, 

 the other seaward. This explains the frequent predomi- 

 nance, along the shore and in cei-tain local portions of fossil- 

 iferons beds, of one valve, the other being often entirely 

 absent or at least very rare. 



The marine pelecypod normally passes through a mero- 

 plaidvtonic larval stage — the Trochophore — in which the 

 young is provided ^^■ith a velum, furnished with vibratory 

 cilia' (veliger stage). At certain seasons of the year these 

 ciliated embryos swarm in the pelagic district, especially in 

 the neighborhood of the shores, whei-e they become the sport 

 of the currents, which distribute them far and wide. When 

 they finally settle down on the sea bottom, upf)n the loss of 

 the velum, they will develop further if they reach the proper 

 substratum, other conditions being favorable. Vast num- 

 bers of the larvee are destroyed before they reach the bot- 

 tom, serving as food for all kinds of animals, or succumbing 

 to unfavorable conditions, and vast numbers of others die 

 from falling on an unfavorable bottom. That most species, 

 nevertheless, develop to the fullest extent is due to the enor- 

 mous fecundity of most pelecypods. As an extreme exam- 

 ple, mayperhaj^s be mentioned our common northern 03'ster, 

 Ostrea vivghiiana, which, according to Brooks," produces 

 nine millions of eggs. In fresh-water pelec^q^ods the niero- 

 planktonic veliger larva exists in one species only (Dreissen- 

 sia polvmorphci), yvhich is said to have migrated from salt 

 to fresh water in recent geologic times. (Lang.) In the 

 other fresh- water pelec^'pods the development proceeds in a 

 different manner — special adaptations to special modes of 

 life beii>g met with. In some cases ( Pidium, Cyclas ) the eggs 

 develop in special brood-capsules in the gills of the mother, 

 leaving these with shell fully formed, as young bivalves. In 

 these genera the velum remains rudimentary, the animal 



* Arch. Z06I. Experim., IX., p. 28. 



