CHAPTER II 

 MORPHOLOGY AND STRUCTURE OF SHELL 



Page 



Appearance and principal axes _ _ 16 



Dimensions. ___ _ __ 'jO 



Shape of shells _ 21 



Growth rings and growth radii _ _ _ 26 



Changes In the direction of principal axes of shell. 27 



Dimensional relationships of shell 29 



Shell area 30 



Chalky deposits _ __ 32 



Chambering and blisters 35 



Structure of shell 36 



Organic material of the shell 37 



Muscle attachment 41 



Chemical composition 43 



Bibliography 45 



APPEARANCE AND PRINCIPAL AXES 



The body of the oyster is covered with two 

 calcareous valves joined together by a resilient 

 ligament along the narrow hinge line. The valves 

 are slightly asymmetrical. The left one is larger 

 and deeper than the right one, which acts as a lid. 

 Under normal conditions the oyster rests on the 

 left valve or is cemented by its left valve to the 

 substratum. The difference between the riglit 

 (flat) and left (cuplike) valve is to a certain degree 

 common to all the species of oysters which have 

 been sufficiently studied. Orton's (1937) state- 

 ment with reference to Ostrea edulis that: "In 

 life the flat or right valve usually rests on the sea 

 bottom and is often referred to as the lower one" 

 is an obvious oversight. 



In C. virginica the left valve is almost always 

 thicker and heavier than the right one. When 

 oysters of this species are dumped from the deck 

 of a boat and fall through water they come to rest 

 on their left valves. I observed this many times 

 while planting either small oysters not greater 

 than 2 inches in height, or marketable adults of 5 

 to 6 inches. In the genus Ostrea the difference 

 between the two valves is not great, it is greater 

 in the genus Crassostrea, and extremely pronounced 

 in the oyster of uncertain systematic position 

 from Australia which Saville-Kent (1S9.3) has 

 called "Ostrea mordax var. cornucopiaeformis." - 



- I am indebted to H. B. Stenzel for calling my attention to this species and 

 for several suggestions regarding the morphological terminology used in this 

 chapter. 



16 



The oyster is a nearly bilaterally symmetrical 

 mollusk with the plane of symmetry passing be- 

 tween the two valves parallel to their surfaces. 

 In orienting any bivalve it is customary to hold 

 it vertically with the narrow side uppermost (fig. 

 15). The narrow end or ape.x of the sheU is called 

 the umbo (plural, umbos or umbones) or beak. 

 A band of horny and elastic material, the ligament 

 (fig. 16) joins the valves at the hinge on which 

 they turn in opening or closing the shell. 



In many bivalves the hinge carries a series of 

 interlocking teeth, but these structures are absent 

 in the family Ostreidae. The hinge consists of the 

 following parts: a projecting massive structure 

 within the right valve, the buttress, according to 

 Stenzel's terminology, supports the midportion of 

 the ligament and fits the depression on the left 

 valve. The tract made by the buttress during 

 the growth of the shell along the midportion of the 

 ligamental area is the resilifer. On the left valve 

 the resilifer is the tract left on the depression. 

 The central part of the ligament is called resilium. 



The pointed end of the valve or the beak repre- 

 sents the oldest part of a shell. In old individuals 

 it reaches considerable size (fig. 17). The beaks 

 are usually curved and directed toward the 

 posterior end of the mollusk although in some 

 specimens they may point toward the anterior. 

 In the majority of bivalves other than oysters 

 the beaks usually point forward. The direction 

 and degree of curvature of the beaks of oysters as 

 well as their relative proportions vary greatly 

 as can be seen in figure IS, which represents 

 different shapes found in old shells of C. virginica. 

 Very narrow, straiglit, or slightly curved beaks of 

 the kind shown in figure 18-1 are usually formed 

 in oysters which grow on soft, muddy bottoms. 

 Extreme development of this type can be seen in 

 the narrow and slender oysters growing under 

 overcrowded conditions on reefs (fig. 19). Other 

 forms of beaks (fig. IS, 2-4) cannot be associat- 

 ed with any particular environment. In fully 



FISHERY bulletin: VOLUME 64, CHAPTER II 



