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Early events of fertilization are described in Chaetopleura apiculata and other selected Chitonida. C. apiculata egg hulls are elaborated into multi-branched spines with interlocking polygonal bases. Around the perimeter of each base are a series of open pores, ranging in size from 0.1-0.5 µm, which permit sperm direct access to the vitelline layer. In Callochitonidae (Chitonida) even larger pores occur in egg jelly coats, but this is considered to be the plesiomorphic condition, found also in Lepidopleurida such as Deshayesiella curvata. Other Chitonina, such as Rhyssoplax tulipa and Acanthopleura granulata, have a continuous outer dense layer that lacks pores and must be digested by penetrating sperm. Fertilization in Chitonida is unique and involves injection of chromatin into the egg via a narrow tubular nuclear extension that appears to exclude other sperm organelles, including mitochondria, centrioles, and flagellum. New evidence from studies of fertilization in Mopalia muscosa (Chitonida: Acanthochitonina) supports this hypothesis. This type of fertilization implies maternal inheritance of both mitochondria and centrioles, which is highly unusual, because in most animals one sperm centriole assists movements of pronuclei and regulates organization of the mitotic spindle. This mechanism of fertilization is defined by a series of apomorphic characters that unify the order Chitonida. [ABSTRACT FROM AUTHOR]