One of the most elusive problems in biology has been documenting the early history of the Metazoa, particularly the origin and emergence of the Bilateria (Wilmer 1990; Conway Morris 1993). The fossil record has been little help in resolving this puzzle because so few primitive Bilateria, for example Platyhelminthes and Nematoda, are preserved as fossils (Conway Morris 1993). However, trace fossils (such as tracks or burrows preserved in bedding planes) indicate that the Bilateria emerged as tiny worm-like organisms about 600-900 Ma BP (Crimes 1992,1994). Boaden (1977) in his 'Thiozoon' model proposed that early metazoans were microscopic anaerobes which evolved in shallow water anoxic, sulphur-rich sediments. Among extant Metazoa, only a select few microscopic organisms (including turbellarians, gnathostomulids, gastrotrichs and nematodes) are capable of inhabiting this niche (Fenchel & Riedl 1970; Boaden 1975). Macroscopic size among Metazoa was achieved only after this period (Boaden 1975; Knoll 1992), and may have been coincident with a rise in atmospheric oxygen levels beyond a critical threshold for sustaining microscopic life (Knoll 1191, 1992; Derry et al. 1992). Small body size and lack of a coelom among extant organisms, disallows expansion of gonads and, in general, precludes reproduction by external fertilization (Olive 1985; Rouse & Fitzhugh 1994). Rather, Microscopic early bilaterians may have employed internal fertilization with direct sperm transfer, as occurs in all extant Platyhelminthes, whose ancestors are now considered basal to the Bilateria (Ax 1985)  

Sponges and cnidarians, which are primitive metazoans (based on molecular evidence, see Lake 1990), fertilize externally with sperm that have the typical shape of a round head, four or five spherical mitochondria surrounding a pair of centrioles and a long tail with 9+2 arrangement of microtubules (Franzen 1956). In keeping with Franzen's original hypothesis that sperm structure correlates with biology of fertilization (Franzen 1956, 1970) but to avoid previous connotations of 'primitive' and 'modified' being associated with sperm shape, Rouse & Jamieson (1987) devised a new system of nomenclature. This new system more accurately describes the functional qualities of sperm: 'ectaquasperm' characterize external fertilizers; 'entaquasperm' are found in internal fertilizers, in which the sperm swim in seawater before reaching the eggs; and 'introsperm' typify internal fertilizers, in which the sperm never contact the water.  

Sperm structure has been thought to be fairly conservative in an evolutionary sense and is now widely used to aid phylogenetic reconstructions (Jamieson & Rouse 1989; Justine 1991; Wirth 1991; Buckland-Nicks 1995). For this reason, we examined spermiogenesis of the neomenioid mollusc, Epimenia australis (Thiele 1897) (Aculifera: Neomeniomorpha) which, based on morphological characters (Scheltema 1993) as well as molecular analyses (Runnegar 1995), is considered basal to the Mollusca. Hence it can be expected that living neomenioids possess some sperm characteristics that are archetypical to their molluscan descendants. This paper includes the first detailed description of spermiogenesis in Neomeniomorpha, which are the only group of aculiferan molluscs that fertilize internally.