THE CLAM AND OTHER BIVALVES 



167 



FIG. 86. Oyster Larva. Much 

 enlarged. (After Mcebius) 



1, mouth; 2, stomach; 3, aims; 

 4, shell ; 5, adductor muscle ; 

 0, circle of cilia 



Development. In the months of May, June, and July, in 

 the latitude of Baltimore, the male and female oysters send 

 out into the water their spermatozoa and their eggs. Pro- 

 fessor Brooks, of Johns Hopkins 

 University, estimates the number 

 of eggs which a female oyster 

 may yield in one season to be at 

 least nine million. "The number 

 of male cells," he says, " is great 

 beyond all powers of expression." 

 Of course we should expect that 

 in spite of the countless sperma- 

 tozoa, many eggs would never be 

 fertilized at all, on account of 

 their being carried away by un- 

 favorable currents. Eggs that are 

 not carried away fall to the bot- 

 tom naturally. Those eggs which are fertilized swim to 

 the surface as larvae after a few hours' development. There 

 surface-fish may, as Professor Brooks suggests, " gulp down 

 in a few seconds oysters equal in number to the population 

 of Baltimore." 



Within one to six days after fertilization the oyster " fry " 

 (swimming larvae, Fig. 86) sink to the bottom again and affix 

 themselves by the margin of the left mantle-fold to whatever 

 solid object they happen to touch. At this time they are 

 about .3 mm. (^ in.) long. For a few weeks after beginning 

 their stationary life they are liable to be crunched to death 

 by voracious crabs, among others, the blue crab (Fig. 71). 



Economic Importance. The largest and most important 

 " oyster-farms " along our coast are in Chesapeake Bay. 

 There and elsewhere the beds have been surveyed and leased 

 under laws of the states. So vast is the oyster-fishing industry 

 in this country that millions of dollars are paid annually by 



