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“Embryos of the freshwater common pond snail Lymnaea stagnalis complete direct development, emerging as shell-bearing hatchlings approximately 10 ?days post-oviposition in a low-ionic strength, minimally buffered environment. In the absence of ambient total calcium (Ca), they tend to develop at a retarded rate relative to controls but grow and hatch at least as well as controls when reared in carbonate-free water. Although they are provided with maternal stores of calcium and carbonate in the perivitelline fluid and within the gelatinous matrix (tunica interna) of the egg mass, they begin to rely on Ca2+ from the surrounding medium as maternal stores in the perivitelline fluid and tunica capsulis become depleted, at approximately day ?5 post-oviposition. At this point in embryonic development, embryos increase oxygen consumption and total CO2 retention…The post-metamorphic Ca2+ requirements seem to be met entirely by active uptake, but the carbonate requirements can apparently be accommodated through both uptake and endogenous sources or by endogenous sources exclusively.” (Ebanks et al. 2010:4092)

“The system for Ca2+ and carbonate acquisition in the freshwater common pond snail Lymnaea stagnalis exhibited sensitivity to several pharmacological agents, which indicate that both Ca2+ uptake from the environment as well as endogenous HCO3– and/or CO32– production work together directly and indirectly for the post-metamorphic embryo to acquire the necessary ions for calcification in freshwater…In times of shell accretion, Ca2+ is taken up from the surrounding medium or from food and into the blood for transport to fresh tissues, particularly the mantle, for deposition as CaCO3 after combining with endogenously produced HCO3–. Internal production of HCO3– by the hydration of CO2, also produces H+ ions that are released from the tissues, transported by the blood to excretion sites on the epithelial membrane, and can be exchanged for Ca2+. During times of Ca2+ depletion, Ca2+ can be acquired from the shell, taken into the soft tissues and blood for transport to sites in need to Ca2+ supplement. Thus in adult Lymnaea, Ca2+ is transported between ambient media, fresh tissues, and shell via the blood…In the absence of external HCO3–, Lymnaea stagnalis embryos grow, form shell, develop normally, and hatch in the same time intervals as controls. These observations point to an internal source of HCO3–/CO32– for shell formation via the hydration of endogenous CO2. For this hydration reaction to allow for CO32– accumulation within the organism and for shell formation, hydrogen ions resulting from the hydration reaction must be eliminated. Thus the H+ pump plays a significant role in Ca2+ flux in post- metamorphic stages. By electrogenic excretion of H+ ions, the electrical potential across the apical membrane, and thereby the electrochemical gradient for Ca2+ uptake is maintained.” (Ebanks et al. 2010:4095-4097)