Systematic significance of reproductive characteristics
The southeastern helicarionid radiation is characterized by several morphological characters, including the absence of an epiphallic caecum, a flagellum producing a spinose spermatophore, and a short or absent vagina (Hyman and Ponder, 2010). The function of the epiphallus and epiphallic flagellum is to produce the spermatophore, the tail-pipe forming in the flagellum and the capsule in the epiphallus. The epiphallic caecum is believed to function in turning the spermatophore so that the tail-pipe is moved into the penis first (Dasen, 1933; Van Mol, 1970). In this group, the absence of the epipahllic caecum may be compensated for by the presence of a spermatophore with a relatively short tail-pipe. In Australian helicarionids that do have an epiphallic caecum, a considerably longer flagellum (and therefore a longer spermatophore tail-pipe) is present (e.g., Nitor, Fastosarion, Westracystis; Hyman and Ponder, 2010). There are also a number of species that lack both the epiphallic flagellum and the epiphallic caecum (e.g., Levidens, Sheaia, Tarocystis, Echonitor, Periclocystis). This may indicate a correlation between the presence of the epiphallic caecum and the length of the spermatophore tail-pipe in Helicarionidae.
The presence of the epiphallic flagellum and caecum characterizes the superfamily Helicarionoidea (Helicarionidae, Ariophantidae, Urocyclidae), but both organs have probably been repeatedly lost in all three groups (Hausdorf, 1998; Hyman and Ponder, 2010).
Similarly, genital characters, such as flagellum, epiphallus, and penial sheath, have repeatedly been lost (and possibly been regained occasionally) by members of Camaenidae (Köhler and Criscione, 2015). These findings highlight the evolutionary plasticity of land snail genitalia in general and underpin the conclusion that while usually useful at lower taxonomic levels, they should be used with care and in conjunction with other characters when defining broader taxonomic groups.