Spicules are mineral-based biocomposites skeletal structures that are widely distributed among phylogenetically distant groups of invertebrates (Porifera, Cnidaria, Mollusca, Echinodermata). Subepidermal spicules are formed under the ectodermal epithelium and are characterized for all groups except mollusks (Aplacophora, Polyplacophora, Bivalvia), their spicules are located on the surface of the body. However, one group of mollusks (Gastropoda: Heterobranchia) have unique subepidermal spicules that have never been detected above the ectodermal epithelium and similarly to those characterized for Porifera, Cnidaria and Echinodermata. Understanding subepidermal spicule formation in mollusks could help solve the question on the origin of spicules. Spicules in nudibranchs have been described for more than 150 years, yet ontogenetic dynamics of spicules have never been studied and the full mechanism of their formation remains unknown. Herein we investigate the spicule formation in different stages of postlarval development of the nudibranch Onchidoris muricata (O.F. Müller, 1776). For the first time, ontogenetic transformations of the spicule complex are described using experiments and different morphological methods. Our studies demonstrate that spicules of O. muricata form in the subepidermal space in early developmental stages immediately after veliger settlement. A single spicule forms inside a huge vacuole within a sclerocyte and remains there throughout the entire life of the specimen. Signs of spicule or sclerocyte migration under the epithelium in postlarval development was not found. Spicules only form during larval settlement, increasing only in size as development furthers. For the first time, spicule mineralization zones were detected at the tips of the spicules as well as the presence of collagen I in the overall composition of the spicules. Thus, our findings suggest that spicules form by an ectodermal cell that emerged under the ectodermal epithelium during the earliest stages of postlarval development.