the book, "Oysters," by C. M. Yonge, which 

 is an important general reference. Most of the 

 following material on anatomy and physiology 

 of oysters is based on Yonge 's summary. 



The oyster shell consists of two valves held 

 together at the hinge by a complex elastic 

 ligament. The upper valve normally is flat, 

 while the lower is concave, providing space 

 for the body of the oyster. The concave shell 

 is the one by which the oyster is normally 

 attached. The valves fit closely together, mak- 

 ing a watertight seal when the oyster closes, 

 provided the edges of the shells have not been 

 broken off or otherwise damaged. 



The shell is principally limestone (calcium 

 carbonate) and, therefore, is quite heavy. The 

 formation and repair of the shell are functions 

 of an organ called the mantle, which surrounds 

 the body of the oyster (fig. 2). The umbo, at 

 the hinge end, is the oldest part of the oyster 

 shell. As the oyster grows, the mantle secretes 

 successive layers of shell material, each 

 projecting beyond the previous one. This 

 secretion finally results in a succession of 

 concentric lines marking the external surface 

 of the oyster shell. 



Basically, the oyster shell consists of three 

 layers. The thin outer layer of organic nature, 

 known as the periostracum, protects the cal- 

 careous shell during its formation. Because 

 it is thin and sometimes quite eroded, this 

 layer is often overlooked. The inner epithelium 

 of the outermost fold of the mantle edge 

 secretes the oyster's periostracum. 



Under the periostracum are second and 

 third layers of shell. Both have an organic 

 matrix but are largely calcareous. The second 

 is the prismatic layer, which consists chiefly 

 of crystals of calcium carbonate, a linnelike 

 substance. The prismatic layer, secreted by 

 the outer epithelium of the outer fold of the 

 mantle edge, also shows definite concentric 

 markings of successive periods of growth. 

 The third or innermost layer is sometimes 

 called the nacreous layer and consists prin- 

 cipally of thin sheets of calcium carbonate 

 covering the inner shell surface. This surface 

 usually is smooth and white except for an area 

 of purple scar where the shell muscle is 

 attached. The pearly inner layer of the shell 

 is laid down by the entire mantle surface and, 

 as a result, is lustrous and smooth, showing 

 no growth rings. 



The ligament, which is formed by a spe- 

 cialized gland at the edge of the mantle, 

 consists of the same three shell layers but 

 with the periostracum always worn away and 

 the other two layers not calcified. It is con- 

 tinually being eroded above and being added 

 to below, the latter process being the greater 

 so that it thickens with age. 



The chemical composition of oyster shells 

 may vary from area to area and sometinnes 

 with individuals of different ages. In general, 

 however, the oyster shell contains from 93 



to 95 percent calciunn carbonate and about 0.5 

 percent organic matter. Magnesium carbonate 

 and calcium sulfate also are present m small 

 quantities. Old oyster shells are used for a 

 variety of purposes, including chicken feed. 

 Most of them, however, are saved and replanted 

 later on the oyster beds to "catch" the new 

 generation of oysters. 



The main sections of the soft part of the 

 oyster can be seen if one of the shells is 

 removed (fig. 2). A thin, creamy, membrane- 

 like organ called the mantle lies against the 

 inner sides of both valves. The two principal 

 functions of the mantle are to protect the more 

 vital organs and to secrete the shell. 



Pearls are produced by the oyster mantle, 

 but in the Annerican oyster they are valueless 

 because they lack lustre and usually are mis- 

 shapen. Pearls are small deposits formed 

 around a nucleus, such as the cyst of a para- 

 site, a particle of broken shell, or even a 

 grain of sand. Usually pearls are embedded 

 in the mantle or are found just under the sur- 

 face of the meat; however, they are occa- 

 sionally formed as blisters attached to the 

 inside surfaces of the shell. 



Sometimes thin layers of greenish or brown- 

 ish color are found embedded in the calcium 

 material of the inner shell surface. These 

 layers, which differ in size, are of organic 

 nature, being composed of material known as 

 conchiolin. These layers may be secreted by 

 the oysters as a defense against the intrusion 

 of boring sponges, worms, or other enemies 

 attacking the shells. These layers, however, 

 may be found in oysters not infested with these 

 forms, 



A common characteristic of the oyster shell 

 is the occurrence of soft "chalky" material 

 embedded in the harder inner layers. These 

 deposits may be used by the oysters merely 

 as a measure of economy in smoothing out 

 the inner surfaces of the shells, because 

 depositing "chalky" layers requires only one- 

 fifth as much material as is needed to build 

 the same volume of shell of the usual hard, 

 subnacreous layers. Normally, these deposits 

 are laid down in summer and later in the 

 season are covered \vith harder layers. 



The mantles of oysters have brown, black, 

 or orange margins. The margins regulate the 

 flow of water entering through the shell valves 

 when they are open. The edges of the mantle 

 have many tentacles that perform a variety of 

 functions, including straining the water to keep 

 coarse particles from entering the delicate 

 filtering system of the oysters and warning 

 the oyster of chemical changes inthe surround- 

 ing water. The tentacles are also sensitive to 

 light, enabling the oyster to detect an approach- 

 ing predator when its shadow falls across the 

 mollusk. 



Near the center of the body is a large ad- 

 ductor muscle, attached to both valves, which 

 controls the closing and opening of the shell. 



