The Oysters 



is the heart. In a live oyster you may see it beat! Forward 

 of the gills are two small flaps, the palpi or lips. Between them 

 the large mouth leads to the stomach and intestine. After wind- 

 ing through the body substance this alimentary tube ends in 

 the mantle cavity just above and behind the adductor muscle. 

 The dark mass surrounding the stomach is the liver. The kid- 

 neys and reproductive organs lie close to it, in the body mass. 



Normally, an oyster lies with its two valves slightly ajar. 

 Note the rubbery ligament inside the hinge. When the oyster 

 hears a noise or sees a shadow that suggests danger, the adductor 

 muscle contracts, pinching the ligament and shutting the shell 

 tight. It cannot stay closed long at a time. 



There is a constant current of water flowing into the front 

 of the shell, between the mantle and the body, bathing the gills. 

 This vigorous current is produced by microscopic cilia (hairs) 

 that cover the gill surfaces, and have the strange property of 

 moving rhythmically with oar-like stroke, all acting in unison. 

 The water passes through small pores into the tubular substance 

 of the gills, where the blood is oxygenated. 



Food particles, with which the seawater is laden, are wafted 

 along between the gills, but they do not enter the pores. They 

 accumulate in windrows just outside, and reaching the lips, they 

 are urged on by labial cilia into the mouth. Thus an oyster 

 feeds as it breathes. 



The oyster's eyes are obscure pigment spots in the mantle 

 margins. Feeling, a well-developed sense, is also located in the 

 mantle. Knots of nervous matter connected by threads constitute 

 the oyster's simple nervous system. This is all a headless, foot- 

 less mollusk needs. 



The "beard," which certain oyster recipes require to be 

 removed, is the gills. The "heart," so-called, is the adductor 

 muscle. 



Life History. — In early summer an oyster's gill chambers 

 become gorged with a milky fluid. The females contain ova, 

 the males milt. These two reproductive elements are discharged 

 when ripe, and their unison (the fertilization of the ova) occurs 

 in the water, which soon swarms with fertile eggs. It is estimated 

 that a large oyster can produce sixty million eggs. The average 

 is probably twenty-five million or less. 



The swimming embryo is a somewhat spherical body, one- 



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