Dart formation in Helix aspersa (Mollusca, Gastropoda)

Summary

Dart formation in Helix aspersa has been investigated by SEM of isolated darts at progressive stages in their development, and by histology of dart sacs at the same times. Dart formation begins at the tip of a tubercle where a small group of epithelial cells secrete an organic material filling a small CaCO₃ cone that is the first mineralized part of the shaft. Subsequent secretory activity by an increasing area of the tubercle epithelium results in an increase in the diameter and anterior lengthening of the shaft. Continued secretion by the tubercle and dart sac epithelium produces the flare and finally the corona. A pattern of deposition is also evident in the fine structure of the mineral. In the shaft and vanes there is an inner layer of spherulitic prismatic structure which is covered by a layer of irregular patches of simple prismatic structure. The outermost layer of the shaft and vanes has a continuous simple prismatic structure. Two layers are present in the flare, an inner granular amorphous layer and an outer spherulitic prismatic layer. The corona consists of a single rarefied prismatic layer. A mechanism of dart formation is suggested that involves two types of organic matrix, calcifying and non-calcifying. Measurements of the calcium content of darts, dart sacs, and collars indicate that the hemolymph is the probable source of calcium for the dart.