(Korringa, 1951 ; Orton, 1937; Orton and Amirtlm- 

 lingain, 1927; Worsnop and Orton, 1923), wlio 

 agree that chambering is caused by the shrinkage 

 of the body, withdrawal of shell-forming organ, 

 and deposition of partitions. Salinity changes 

 were suggested by Orton as one of tlie principal 

 causes of chambering, and shrinkage due to 

 spawning was also considered by Korringa as a 

 probable factor. These conditions have not been 

 reported for C. mrginica. I did not find any 

 evidence that chambers or blisters in the American 

 oyster are associated with shrinkage or other 

 body changes. 



It is interesting to add that some taxonomists 

 of the middle of the past century (Gray, 1S33; 

 Laurent, lS39a, lS39b) were so puzzled by the 

 presence of chambers that they compared cham- 

 bered oyster with Nautilus and even suggested 

 the possibility of some family relation between tlie 

 latter genus and Ostrta! 



An interesting sliell structure consisting of a 

 series of cluimbers near the hinge end is found in 

 the Panamanian oyster, 0. iridescens. The loca- 

 tion of chambers and the regularity at wiiich tiiey 

 are formed as the shell grows in height can be seen 

 in figure 43 representing a longitudinal section of 

 the valve made at a right angle to the hinge. 

 This type of chambering is obviously a part of a 

 structural plan of tlie shell and is not a result of an 

 accidental withdrawal of the oyster body or of an 

 invasion by commensals. Arch-forming septae 

 of the chambers apparently contribute to the 

 strength of the hinge and at the same time require 



relatively small amounts of building material. 

 Wliat advantage 0. Iridescens obtains from this 

 type of structure is of course a matter of specu- 

 lation. 



Ciianibers found in C. rirgiiiica consist of 

 irregular cavities containing mud or sea water. 

 Sucli formations are called blisters. Blisters can 

 be artificially induced b}' inserting a foreign 

 object between the mantle and the shell (see p. 105). 

 They are also caused by the invasion of shell 

 cavity by P(di/d<ira (see p. 422) or by perforations 

 of the siiell by boring sponges and clams (p. 420). 



STRUCTURE OF SHELL 



For more than a Imndred years the structure of 

 the molluscan sliell was an object of research by 

 zoologists, mineralogists, and geologists. Several 

 reviews of the voluminous literature (Biedermann, 

 1902a, 1902b; Bciggild, 1930; Cayeu.x, 1916; Haas, 

 1935; Korringa, 1951; Schenck, 1934; Scldoss- 

 berger, 1.S56) deal with the problem from different 

 points of view. Recently these studies have been 

 extended by the use of X-ray and electron micro- 

 scope. The methods, especially those of electron 

 microscopy, opened entirely new approaclies par- 

 ticularly witli reference to the structure of the 

 organic constituents of the shell (Gregoire, 1957; 

 Gregoire, Duchateau, and Florkin, 1950, 1955; 

 Watabe, 1954). 



Terminology of molluscan shells is somewhat 

 confusing depending whether the emphasis is 

 placed on morphological, crystallographical, or 

 mineralogical properties. The names of different 







J I 



Centimeters 



Figure 43. — Shell of (>. iridrsrcrm put at right angle to the hinge. Note a series of empty chambers at the hinge area. 



Specimen from the Gulf of Panama. 



36 



FISH AND WILDLIFE SERVICK 



