Reference: Bin/. Bull 184: 57-78. (February, 1993) Aplacophora as Progenetic Aculiferans and the Coelomate Origin of Mollusks as the Sister Taxon of Sipuncula 1 AMELIE H. SCHELTEMA \Vootls Hole Oceanographic Institution, Woods Hole, Massachusetts 02543 Abstract. Evidence is presented in support of the fol-lowing phylogenetic hypotheses: ( 1 ) Sipuncula are the sis-ter taxon of Mollusca; (2) the two aplacophoran taxa, Neomeniomorpha (= neomenioids) and Chaetodermo-morpha (= chaetoderms), are monophyletic with a com-mon neomenioid-like ancestor, and of the two taxa, Chaetodermomorpha are more derived; (3) Aplacophora and Polyplacophora are sister taxa and form a clade, Acu-lifera; (4) Aculifera are the sister group of the remaining extant mollusks, Conchifera; and (5) Aplacophora are progenetic Aculifera. The evidence is based on homologies of early and late emhryological development, adult morphologies, and molecular analyses. Embryological development in si-punculans and mollusks shows a close relationship be-tween them, and embryological development of the shell separates Aculifera and Conchifera. Adult morphologies indicate: ( 1 ) monophyly of Aplacophora; (2) sister-group relationship between Aplacophora and Polyplacophora; (3) a molluscan plesiomorphy of nonsegmented serial replication of organs; and (4) progenesis in Aplacophora. Molecular evidence supports the embryological and mor-phological relationships between Sipuncula and Mollusca. Mollusca are thus hypothesized to be coelomate Eu-trochozoa, which share an ancestor that probably had se-rial replication of organs. Differences in size and structure of the coelom among Eutrochozoa are hypothesized to have been brought about by changes in the timing and the process of cavitation of the mesodermal bands that arise from cell 4d. Through the process of progenesis Aplacophora retained an ovoid embryological shape and Received 19 August 1992; accepted 25 November 1992. ' Contribution Nos. 8205 from the Woods Hole Oceanographic In-stitution, and 314 from the Smithsonian Marine Station at Link Port. several internal structures that, although they appear to be in a primitive state, are actually secondarily derived as is quadrant D specification during early cleavage. Introduction The uniqueness of Aplacophora among Mollusca lies in their derived vermiform body in combination with an internal organization that appears to reflect a primitive molluscan state, especially the simple ladderlike nervous system, serial musculature, distichous radula (two teeth per row) in its plesiomorphic aplacophoran state, simple digestive system, and epidermis that produces an aculif-erous cuticle. Their evolutionary significance to the phy-lum has long been a matter for conjecture. First came the question of whether Aplacophora were even mollusks, as they lack a number of "typical" characters such as a shell, mantle, and kidneys (e.g., Thiele, 1902; H. Hoffmann, 1929-30), but they have more usually been considered to belong within the phylum because of similarities to chitons in their nervous system ( Amphineura) and spicules (Acu-lifera) (e.g., Spengel. 1881; Heath, 1911). Further discus-sions were concerned with whether aplacophorans were "degraded" or truly "primitive" mollusks (see Hyman, 1967, pp. 68-70 for a historical account). There have been no current arguments which separate Aplacophora from Mollusca since evidence for a close relationship between Aplacophora and Polyplacophora was published by S. Hoffman (1949), but under present discussion is their origin and position within the phylum (Salvini-Plawen, 1972. 1981a, 1985; Scheltema, 1978, 1988), as well as the origin of the phylum Mollusca itself. Mollusca have been argued either to have a noncoelomate origin and to be the sister taxon of the eucoelomate An-nelia-Echiura-Sipuncula (Salvini-Plawen, 1972, 1985 fig. 42), or to be eucoelomates with an ancestor in common 57
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