Centimeters 



Figure 92. — New shell growth formed during 1 year along 

 the periphery of the valve of an adult oyster from Long 

 Island Sound planted in the Oyster River, Chatham, 

 Mass. The newly formed shell is recognizable by zigzag 

 lines of the material: its width is greatest along the 

 ventral edge. 



entire outer surface of the mantle and is not re- 

 stricted to the periostracal groove. Such secre- 

 tion, first observed in pearl oysters (B0ggild, 

 1930), can be experimentally demonstrated in C. 

 virginica. Oysters with one valve removed and 

 the edges of the mantle cut ofT above the peri- 

 ostracal groove secreted a new conchiolin layer 

 over the entire surface of the exposed mantle 

 within 5 days. Although the operated specimens 

 remained alive in the laboratory tanks at Woods 

 Hole over 3 weeks this conchiolin membrane 

 remained uncalcified. In another experiment 

 three adult oysters were removed from their shells 

 and kept alive in sea water for 3 weeks. They 

 formed rather thick coats of periostracum which 

 was very lightly calcified. The repair of holes 

 made in oyster shells by boring snails and sponges 

 also shows that conchiolin is secreted by the entire 

 surface of the mantle. The damaged area is rapidly 



THE MANTLE 



733-851 0—64 7 



covered by a layer of organic material which 

 later becomes calcified. 



Soon after being secreted, the conchiolin be- 

 comes calcified. Progressive stages of this process 

 can be observed on the growing edge of the shell, 

 or by inserting pieces of plastic or small glass cover 

 slips between the edge of the mantle and the valve 

 and removing them at regular intervals for in- 

 spection. The earliest stage of calcification is 

 recognized by the appearance of minute granules 

 of calcium salts, which become visible in polarized 

 light as brightly sparkling dots (fig. 93) . At this 

 early stage the distribution of the granules (cal- 

 cospherites) does not show any definite pattern 

 or arrangement. In a living oyster they can be 

 found entangled in strands of mucus left on the 

 conchioUn sheet by the back and forth movements 

 of the mantle edge. Within the next 24 to 48 hours 

 typical hexagonal crystals of calcite can be seen 

 (fig. 94, black crosses). They gradually increase 

 in size and present a picture of great brilliance 

 and beauty in polarized light (fig. 95) . 



Distribution of calcospherites at the stage of 

 their transformation into small calcite crystals on 

 the surface of the newly secreted shell (fig. 96) 

 does not show any distinct orientation in relation 

 to the growth axis of the shell. Some of the cal- 

 cospherites are scattered over the entire field of 

 vision, while others are packed tightly between 

 the larger crystals (see large group of crystals at 

 the lower part of figure 96). Within the next 48 

 hours the calcite crystals increase in size (fig. 97) . 

 In the final stage of shell formation the calcite 

 crystals become arranged in a distinct pattern to 

 form the prismatic layer in which each unit is a 

 prism oriented with its long axis at about a 90° 

 angle to the edge of the shell (fig. 98). The form 

 of the individual prisms varies greatly, some of 

 them are even wedge-shaped and slightly curved. 

 This can be observed after boiling a piece of shell 

 in a strong sodium hydroxide solution to separate 

 the prisms (Schmidt, 1931). 



Each calcite prism is surrounded by a capsule 

 of conchiolin. By dissolving the mineral in weak 

 hydrochloric acid it is possible to obtain intact 

 the organic meshwork of the conchiolin layer. 

 The walls of each capsule, as can be seen in figure 

 99, are very thin and slightly iridescent. Since in 

 the earliest stages of shell formation the conchiolin 

 sheet appears to be amorphous under the light 

 microscope, it is reasonable to assume that the 

 organic capsules of the calcite prisms are formed 



93 



