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Sperm structure of the Xenacoelomorpha (Acoelomorpha plus Xenoturbellida) is updated in the light of new discoveries or new interpretations of existing data. Nemertodermatida and Acoela (Acoelomorpha) have introsperm with certain basic features in common, but all acoels lack acrosomes and usually have two flagella with unusual combinations of microtubules, whereas all nemertodermatids have small, simple acrosomes and a typical 9 + 2 flagellum. Xenoturbellida is currently considered as the sister taxon to Acoelomorpha. Xenoturbella bocki has an aquasperm that has almost nothing in common with the sperm of Acoelomorpha. We argue that the aquasperm ultrastructure of X. bocki has much in common with sperm of hemichordates and to some extent echinoderms, which was previously disputed. Molecular analyses have on the one hand supported a connection with deuterostomes but on the other hand have negated it, suggesting that the closest common ancestor of Xenacoelomorpha is either the Nephrozoa, Deuterostomia or Protostomia. Sperm structure is highly diverse among Xenacoelomorpha, with protostome-like traits in Acoelomorpha and deuterostome-like traits in Xenoturbella. Assuming Xenacoelomorph monophyly and ancestral introsperm in this taxon, however, suggests that the re-expression of the aquasperm form of Xenoturbella, involving some key changes in sperm morphology, is a secondarily derived state that could have occurred through "progenetic spermiogenesis" with the precocious development of round spermatids to maturity. [ABSTRACT FROM AUTHOR]

Gametogenesis in Chitonida is highly unusual, giving rise to eggs with elaborate, often spiny hulls and sperm with needle-like nuclear extensions and tiny acrosomes. My studies of oogenesis in a variety of chitonid families have revealed unique mechanisms of egg hull formation. Three-dimensional reconstruction of serial sections of oogenesis in the chitonidAcanthochitona garnoti(Blainville) confirms that the egg hull is formed largely by microapocrine secretions, rather than by exocytosis of Golgi vesicles. This novel process, detailed for the first time in Mollusca, bypasses the endomembrane system and releases compounds formed in the cytoplasm directly from the cell surface via budding of ‘microaposomes’. Fertilization is no less unusual. In virtually all Metazoa, except chitonids, the entire sperm enters the egg, and sperm mitochondria are digested. Typically, the centrosome that derives from the sperm centriole acts as a microtubule organizing centre, moving the two pronuclei together to form the zygote nucleus and later initiating cleavage of the embryo. However, in chitonids the sperm digest a minute pore in the egg hull that allows injection of DNA but blocks entry of sperm organelles, including the nuclear membrane. Serial sections of fertilized eggs ofMopalia muscosa(Gould) confirm that a membrane bag, abandoned on the egg surface, contains the sperm organelles. Thus, not only is there maternal inheritance of mitochondria and centrioles in Chitonida but also other sperm proteins normally associated with fertilization may be blocked from entering the egg. [ABSTRACT FROM AUTHOR]