"Archaeogastropoda" is a large gastropod group with a long and successful history from the late Cambrian to the Recent. More than eighty extinct and extant families have been described as archaeogastropods (Knight et al., 1960), and recent members are assigned to about forty families (see Appendix). Anatomically, some archaeogastropods have been thought to exhibit many primitive features within Gastropoda. Especially, symmetry of various pallial and internal organs (ctenidia, osphradia, hypobranchial glands, auricles, and kidneys) has been regarded as one of the plesiomorphies retained from the original molluscan plan e.g. Graham, 1985). Therefore, "Archaeogastropoda" plays an important role in phylogenetic studies as a ey taxon in considering the origin and subsequent radiation of gastropod groups.

The traditional concept of Archaeogastropoda has been greatly modified as a consequence of phylogenetic analyses based on various anatomical characters. (1) Thiele (1925, 1929): Thiele's classical concept of Archaeogastropoda included Zeugobranchia, Docoglossa, Trochaecea, Neritacea, and Cocculinacea, among which Zeugobranchia was considered to be most primitive. (2) Cox (1960), Cox and Knight (1960), Knight et al. (1960): These authors basically followed Thiele's scheme, and divided Recent members into four suborders, Pleurotomariina, Patellina, Trochina, and Neritopsina. (3) Golikov and Starobogatov (1975); Drastic changes in gastropod systematics were proposed by these authors. The prosobranch gastropods were split into three subclasses, Cyclobranchia (including Docoglossa), Scutibranchia (including zeugobranchs), and Pectinibranchia (including Turbinimorpha, Neritimorpha, and caenogastropods). (4) Salvini-Plawen (1980): Vetigastropoda was proposed for Zeugobranchia, Trochoidea, and Cocculinoidea based on the predominance of the right shell muscle. Effectiveness of this original definition was denied by Haszprunar (1985 c), but later the taxon was justified by several synapomorphies. (5) Salvini-Plawen and Haszprunar (1987), Haszprunar (1988 a, b; 1993). and Salvini-Plawen and Steiner (1996): The phylogramsby these authors differ from others in sequential derivation of Docoglossa, Cocculiniformia, Neritopsina, Vetigastropoda, hot-vent taxa (Peltospiroidea and Neomphaloidea), Seguenziina, Architaenioglossa, and the remaining higher groups (Apogastropoda). A hypoathroid-type of nervous system was employed as diagnostic character of Archaeogastropoda, including Architaenioglossa. (6) Hickman (1988): From a phylogenetic (rather than taxonomic) viewpoint, Archaeogastropoda was restricted to Pleurotomarioidea, Fissurelloidea, and Trochoidea. This narrow definition has not been followed by any subsequent authors. (7) Ponder and Lindberg (1996: 1997): A cladistic analysis form all gastropod groups was carried out based on outgroup comparison with testarian taxa. The names "Eogastropoda" (= Patellogastropoda + possible coiled ancestor) and "Orthogastropoda"(=all remaining gastropods) were proposed by these authors to signify explicitly distinct states of two basic clades of Gastropoda. Hot vent-taxa and Vetigastropoda were united as Archaeogastropoda (s. s.).

As is suggested by historical review, one of most significant changes in recent theories is that thepleurotomarioidean-like gastropod ancestor of the traditional view become no longer supported, and Patel logastropoda are regarded as most primitive. It has also been revealed that the relationships among "Archaeogastropoda" are more complex than traditionally known. These advances have been provided mainly by the following innovations in the anatomical approach as noted by Haszprunar (1988 a, b). (1) New material: Totally unknown new taxa of higher rank have been discovered from deep-sea environments, especially from the hydrothermal vents. The range of "Archaeogastropoda" was markedly extended from tradi tional one. (2) Microscopic anatomical observation: Microscopic or submicroscopic techniques have been applied to observations of soft anatomy. Most typically, observations with transmission electron microscopy (TEM) on the osphradial sense organ (Haszprunar, 1985 a, b) and sperm morphology (e.g. Koike, 1985; Healy, 1988, 1996; Hodgson, 1995) revealed a number of new phylogenetic characters. (3) Advances in phylogenetic methodology: Cladistic analysis has been almost universally adopted over a traditional empirical approach (Salvini-Plawen and Steiner, 1996; Ponder and Lindberg, 1996; 1997). Phylogenetic relationships among taxa are interpreted based on the distinction between apomorphy and plesiomorphy, synapomorphy and homoplasy, and monophyly and poly-/paraphyly rather than simply based on the degree of similarity or dissimilarity. Only monophyletic taxa defined by synapomorphic homologues are regarded as acceptable in cladistics.

Several major problems, however, still remain unsolved in gastropod phylogeny as pointed out by Ponder and Lindberg (1996). It is very apparent that further advance will rely upon filling gaps in knowledge of morphological characters by new observations. Biased information on characters known from limited taxa is especially problematic. It is often difficult to fill gaps in a data matrix from information based only on earlier literature. Lengthy classic descriptions are often ineffective, when important characters in current systematics are not described according to required standards. Thus, additional comparative studies covering a wide range of characters and taxa are indispensable for the morphological approach toward resolving relationships.

This study has its aim particularly on the following four subjects, taking the above-mentioned background and problems into consideration. (1) New anatomical observations: In this study, attempts were made to reveal new anatomical information useful for higher phylogenetic studies, (i) Internal anatomy in addition to external anatomy was directly observed with scanning electron microscopy (SEM). This method helps to eliminate artifacts in reconstructing complex three-dimensional structures, (ii) In macroscopic anatomy, special attention was paid to unexplored organs. In particular, comparative anatomy of the buccal mass successfully revealed a number of useful characters. (2) Cladistic analysis: Based on anatomical observations, discrete homologous characters were used for Cladistic analysis. All characters were unordered and unweighted a priori to carry out an "assumption-free" analysis. The relationships of taxa including outgroups were determined by calculation based on parsimony criteria. The analysis was performed on operational taxonomic units (OTUs) at the generic level. (3) Hypotheses of character state transformation: This study also aims to present hypotheses on the pathways of morphological evolution. The polarity of transformation series was traced on cladograms a posteriori to suggest how respective morphological stateshave evolved. (4) Relationships between shell and soft parts: Phylogenetic congruence of protoconch characters was tested by comparing the relationships inferred from non-shell characters.