Results obtained from a histological, autoradiographic, and operative investigation of the radulae of two pulmonates have led to the formulation of the following theory of radula replacement. The radula is secreted continually by the permanent odontoblasts and continually moves anteriorly. The superior epithelium of the radular gland is produced by division of cells near the odontoblasts and moves forward at the same rate as the radula; the cells produce materials that harden the radula and then die. The inferior epithelium is also produced by division of cells near the odontoblasts, but initially moves anteriorly at only a quarter of the rate of the radula. The epithelium then changes shape from a columnar to a pavement epithelium. It is suggested that it then moves forward at the same rate as the radula. The pavement epithelium secretes the subradular membrane which may serve to attach the epithelium to the radula, and the supralateral radula tensor muscles are attached to the basement membrane of this epithelium. At the extreme anterior end of the radula the inferior epithelium becomes again columnar and secretes a cuticle continuous with the buccal cuticle. This process detaches the subradular membrane and the radula from the epithelium. The epithelial cells then die and disintegrate. The detached radula and subradular membrane show a change of staining properties suggestive of reversal of the tanning, and they then break up.

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