t 



Millimeters 



"^.5 



Figure 46. — Cross section of a piece of young shell of C. 

 virginica (mounted in bakelite and ground on a glass 

 wheel with carborundum, about 80 x). Periostracum 

 (top line), prismatic layer (middle), and calcite-ostracum 

 (lower). 



form of thin, homogenous sheets of organic material 

 kept together like pages of a book. Tliis sub- 

 stance, discovered in 1S55 by Fremj^ is known as 

 conchiolin. The name is applied to the organic 

 material insoluble in water, alcohol, ether, cold 

 alkaline hydroxides, and dilute acids. In the 

 literature it appears also under the names of 

 conchin, periostracum, epidermis, and epicuticuhi. 

 Conchiolin is a scleroprotein, the structural for- 

 mula of which has not yet been determined. The 

 elementary analysis of conchiolin of 0. edulis 

 (Schlossberger, 1S56) is as follows; H, 6.5 percent; 

 C, 50.7 percent; N, 16.7 percent. Wetzel (1900) 

 found that conchiolin contains 0.75 percent of 

 sulfur and Halliburton (quoted from Haas, 1935) 

 assigned to it the following formula: C30, H4g, Ng, 

 0,1, which also appears in the third edition of 

 "Hackh's chemical dictionary" (Hackh, 1944). 

 Similarity of conchiolin to chitin - leads many 

 investigators to an error in ascribing cliitinous 



' composition to structures which were found in- 



; soluble in alkaline hydroxides and dilute acids. 



i Thus, the presence of chitin was reported in the 

 shell and ligament of Anodonta, Mya, and Pecten 



'• (Wester, 1910). The application of the Schulze's 



test for chitin (intense violet coloration after 

 treatment for 24 hours in diaphanol [clilorodioxy- 

 acetic acid], followed by a solution of zinc cldoride 

 and iodine), does not confirm these findings (Lison, 

 1953).-' 



To the naked eye and under the light micro- 

 scope the conchiolin appears as amorphous, viscous 

 and transparent material which hardens shortly 

 after being deposited. Using the electron micro- 

 scope technique, Gregiore, Duchateau, and Florkin 

 (1955) found that the conchiolin of gastropods and 

 bivalves consists of a fine network with many 

 meshes of irregular shape and variable dimensions. 

 This is, however, not the case in oyster shells. 

 Conchiolin of the genus Ostrea lacks meshes and 

 under the electron microscope is of uniform ap- 

 pearance (personal communication by Gregoire). 



Cross sections of decalcified shells of C. vircjinica 

 show a distinct difference between the staining 

 properties of the conchiolin of the prismatic and 

 calcite-ostracum layers. On the cross sections of 

 shell shown in figure 49 the two parts can be 

 recognized by the typical foliated appearance of 

 the calcite-ostracum and the meslilike structure 

 of the prismatic layer. In the preparation stained 

 with Malloiy triple dye the organic matter of 

 the walls of the prisms are stained reddish-brown 

 while the foliae of the calcite-ostracum are bluish. 

 Differential staining indicates the dift'erence in 

 the chemical composition of the two parts. 



The amount of conchiolin in the oyster shell was 

 studied by several investigators. As early as 

 1817 Brandes and Bucholz estimated that organic 

 material of the shell constitutes about 0.5 percent 

 of the total weight. Schlossberger (1S56) found 

 6.3 percent of organic matter in the prismatic 

 layer of the oyster but only from 0.8 to 2.2 percent 

 in the calcite-ostracum. According to Douville 

 (1936), the albuminoid content of the oyster shell 

 is 4.8 percent. 



According to the determinations made by A. 

 Grijns for Kt)rringa (1951), the conciiiolin content 

 of the prismatic layer of (). edulis varied from 3.4 

 to 4.5 percent against the 0.5 to 0.6 percent in the 

 calcite-ostracum. The conchiolin content was 

 calculated from the percentage of X (by Kjeldahl 

 method) nmltiplied by 6.9. Tlie results of my 

 determinations of the wciglit of organic material 



* Inasmuch as the same reaction is obtained with cellulose and tunicine, 

 additional tests should he made using Lupol solution and 1 to 2 per cent 

 sulphuric acid (Ii2S04). With this test chitin is colored hrown. while cellu- 

 lose and tunicine are blue. 



MORPHOLOGY AND STRUCTURE OF SHELL 



39 



