10 THE OYSTER AND THE OYSTER INDUSTRY. 



Each valve or shell is lined with a thin membrane called the mantle, 

 fringed on the edge and attached to the shell over nearly all its ex- 

 panse, but free along the margin. The right mantle has been re- 

 moved with the shell in Plate 11. In about the center of the body 

 is the large adductor muscle, by the contraction of which the oyster 

 closes its shell. As stated above, when this muscle is relaxed or 

 cut, the ligament m the hinge forces the valves apart. The main 

 body of the oyster lies between the right and left sides of the mantle 

 and is attached to it and the adductor muscle. It will thus be seen 

 that the oyster is held to the shell by the mantle and the muscle. 



In Plate II part of the body wall is represented as havmg been re- 

 moved with the right side of the mantle, thus exposmg to view the 

 liver or digestive gland and the oesophagus or gullet leading from the 

 mouth to the stomach, wliich opens into the intestine. Tliis extends 

 downward and backward beneath the muscle, then curves sharply 

 and runs forward on the left of the stomach to the oesophagus, 

 where it again turns to the rear and extends backward to the vent 

 or anus above the muscle. 



In a thin-waUed sac, the pericardium, immediately in front of the 

 muscle, lies the real heart of the oyster. (The adductor muscle is 

 often erroneously referred to as the ''heart," since when it is cut 

 the oyster eventually dies from inablility to close its valves.) The 

 circulatory system of the oyster is of the "open" type; that is, the 

 arteries do not terminate in capillaries which lead to the veins, as 

 in man, but deliver the blood, which is colorless, into large spaces, 

 or lacunae, between the tissues. The blood, as it spreads through 

 these lacunae, bathes the various cells of the body and is then gathered 

 up by the veins and collected in the auricle or lower chamber of the 

 heart. It then passes into the ventricle, or upper chamber, which 

 contracts and forces the blood through arteries to the different parts 

 of the body. A hinged valve between the two chambers of the heart 

 prevents the blood being driven back into the auricle. 



The nervous system of the oyster (not shown in the figures) is 

 very simple, consistm^ of two ganglia or knots of nervous matter, 

 lying just over the gullet and two nerves passing back from them, 

 one on each side, to another pair of ganglia beneath the adductor 

 muscle. Smaller nerves extend from these two pairs of ganglia to 

 the various parts of the body. 



At the anterior end of the body four thin lips or palps hang free in 

 the mantle cavity and extend backward from beneath the mouth for 

 about one-third the length of the body, the posterior ends lapping 

 under the ends of the gills. The latter, lour in number, are somewhat 

 similar to the palps in appearance, and extend backward and upward 

 in crescent fashion, as shown in Plate II. Microscopic examination 

 sliows that the gills are covered on both sides with very fine hairs or 

 cilia, arranged in rows. These beat back and forth and, \^•hen the 

 oyster is lying \\ ith the valves open, cause a current of sea water to 

 pass on to the giUs. The water is forced through fine openings on 

 the surfaces of the giUs into water tubes inside the giUs and thence 

 into the cavity above them. As the water passes through the gills 

 the blood is aerated as in the case of a fish. In Plate II the openings 

 of the tubes can be seen on the inner edge of the gills. The right 

 mantle having been removed, the cavity into which the water passes 

 is exposed. It lies in tne space just above the inner edge ol the giUs. 



