692 



HISTORY OF THE SEA. 



from Gravesend, in the estuary of the Thames and midway 

 along the Kentish coast, and in the estuary of the Coluc and 

 other small streams on the Essex coast. The Frith of Forth 

 is also famous for its oyster beds. The product of these beds 

 has diminished in recent times ; according to some authorities 

 from too improvident and persistent dredging, but Mr. Buck- 

 land attributes the decrease in the yield to sudden changes in 

 the temperature at the critical period when the spat, or young 

 oysters, are just formed, rather than to over-dredging 



The United States is more abundantly furnished with oyster 

 beds than any other country. They extend along almost the 

 entire coast. Those of Virginia are estimated to comprise 

 nearly 2,000,000 of acres. The sea-board of Georgia is famous 

 for its immense supplies, while the whole 115 miles of Long 

 Island is occupied with them. 



The oyster is one of the lowest forms of the mollusk. It* 

 mouth opens right into its stomach, which is surrounded b^ 

 its liver, permeated by a yellow liquid, the bile. It may thus 

 be said that they have their stomach and intestine in the liver, 

 the mouth upon the stomach and the opening of the intestine 

 in the back. They have a heart which circulates a colorless 

 blood. They breathe at the bottom of sea, having an organ 

 which separates from the water the small amount of oxygen 

 it contains. Their respiratory organs are two pair of gills, or 

 branchiae, curved and formed by a double series of very deli- 

 cate canals placed close together, resembling the teeth of a fine 

 comb. This apparatus, like the mouth, is hidden under the 

 fold of the mantle. They have no brain, but a ganglion of 

 nerves, a whitish substance situated near their mouths. From 

 this originate the nerves, which branch off to the region of 

 the liver and stomach ; here they re-unite in a second ganglion 

 which is placed behind the liver. The nerves of the mouth 

 and its tentacles originate in the first ganglion, those of the 

 respiratory organs in the second. It has no sense of sighi 



