X-Ray Radiographic Examination of Dart Formation in Helix Aspersa

Formation of the dart in the snail Helix aspersa occurs following loss of the dart during mating behavior. The new dart is formed during a period of 5-6 days within a dart sac lined with epithelium, and the stages of its development can be followed by x-ray radiography. CaCO₃ deposition is initiated at the tip of a conical tubercle covered with epithelium and proceeds anteriorly forming a shaft with blades and then posteriorly with the formation of a flared region bearing pleated rods. The form of the dart appears to be governed by the conformation of the epithelium lining the dart sac. The calcium content of the dart is approximately 0.5 mg. The dart sac tissue (excluding the collar spherules) contains less than 5% of this amount, indicating that the calcium used for dart formation comes from other body or shell sources.