calcite and gypsum (hydrous calcium sulfate), 

 with the latter present only as a minor constituent. 

 The oysters appeared to be normal in every respect 

 and showed good growth of shells. The presence 

 of gypsum is of interest since it is not a normal 

 constituent of oyster shell. What particular 

 disturbance in tlie calcium metabolism produced 

 its formation is unknown. 



SOURCES OF CALCIUM 



It has been suggested (Pelseneer, 1920; Galtsoff, 

 1938) that lamellibranchs may remove calcium 

 directly from sea water. Pelseneer (1920) cites 

 an example of a young Anodonta cy(jnea wliicii in 

 2 months removed all the calcium from 5 1. of 

 water in wliicli it was kept. Definite proof of the 

 direct absorption of calcium by the oyster mantle 

 is given by tlie experiments with ('. i'ir(jinica 

 (Jodrey, 1953) in which radioactive Ca^° was used. 

 Calcium turnover was also studied by Hirata 

 (1953) in mantle-shell preparations made by cut- 

 ting off tiie adductor muscle and the visceral 

 organs, and leaving the intact mantles spread over 

 their respective valves. Tlie mantle remained 

 alive for several days and deposited tlie shell 

 material, although at a lower rate tlian does the 

 intact oyster. Jodrey placed a mantle prepara- 

 tion in 500 ml. of aerated sea water with a Ca" 

 activity of 5.8 microcuries. At least part of the 

 calcium of the newly formed sliell substance came 

 directly from the sea water, and the deposition of 

 calcite took place in tissue isolated from the circu- 

 latory and digestive systems. The experiments also 

 demonstrated tliat the greater portion of calcium 

 in the mantle appears to be inert. Only 2.5 per- 

 cent of the total calcium content was renewed 

 every 24 minutes, the turnover being 0.6 mg. of 

 calcium per minute per gram of mantle. In 

 addition to entering the mantle directly calcium 

 can be taken up by other organs of the oyster and 

 transported to the mantle (Wilbur, 1960). 



MINERALOGY OF CALCIUM CARBONATE 

 IN MOLLUSCAN SHELLS 



Calcium carbonate is known to occur in 12 

 mineral forms (Prenant, 1924), but only three of 

 these have been found in aninuils. In tlie shells 

 of mollusks, calcium carbonate usually occurs as 

 calcite and ai'agonite. There are many species in 

 which both minerals occur together although in 

 different parts of the shell. Prenant (1928), who 

 contributed much to the study of calcification, 

 found that besides calcite and aragonite the animal 



tissue may contain small spheres (sphaerolithes) or 

 tiny needles of the mineral called "vaterite", after 

 tlie mineralogist Vater who discovered it. Vater- 

 ite was reported to be present in the connective 

 tissue of certain gastropod mollusks, cestodes, and 

 trematodes, and in the fat tissue of insects (Dip- 

 tera). Its presence in the tissues of the oyster 

 has not been reported. 



The various forms of calcium carbonate secreted 

 by aninud tissue can be identified l)y their crystal- 

 lograpliic properties, birefringence, density, and 

 chemical reaction. 8ome of these distinctive 

 cliaracteristics are summarized in table 12, taken 

 from Prenant (1924). 



Impurities always present in material secreted 

 by living forms can sometimes make the mineral- 

 ogical identification of calcium carbonate doubtful. 

 Calcite and aragonite can be distinguished by 

 means of tlie polarizing microscope. Calcite 

 crystals examined uiuler crossed nicols give a 

 brilliant jjicture of various colors, and a distinct 

 Ijlack cross appears when the optical axis is aligned 

 parallel to tlie axis of the microscope (fig. 94.) 

 In tlie case of aragonite, hyperbolic arched lines 

 appear instead of the black crosses. Exact 

 identification of minerals can of course be made by 

 X-rays, but this method is rarely available to the 

 biologist. 



Among various chemical identification methods 

 tiie Meigen color reaction can be most easily 

 em])loyed (B0ggild, 1930, p. 238). In a weak 

 solution of cobalt nitrate aragonite becomes violet, 

 the intensity of coloration increasing as tlie solu- 

 tion is warmed. Calcite, however, remains pale 

 blue even in a heated solution. 



The conditions under which a mollusk secretes 

 calcium carbonate in a specific mineralogical form 

 are not at present understood. It is reasonable 

 to presume that the organic matrix of the shell is 

 someliow involved in this process. Roche, Ran- 

 son, and Eysseric-Lafon (1951) found that in the 

 shells of mollusks consisting both of calcite and 

 aragonite the conchiolin associated with tiie calcite 

 of the prismatic layer had higher concentrations 

 of glycine and tyrosine than were present in the 

 nacre of the same shell consisting of aragonite 

 (see cii. II, p. 41). The causal relationship be- 

 tween the mineralogical forms of carbonate and 

 amino acids of its conchiolin lias not been 

 demonstrated. 



A iiypothesis that carbonic anliydrase, an 

 enzyme present in tiie tissues of the mantle, plays 



THE MANTLE 



103 



