MALACOLOGIA, 1978, 17(1): 99-109 POSITION OF THE CLASS APLACOPHORA IN THE PHYLUM M0LLUSCA1 Amélie H. Scheltema Woods Hole Océanographie Institution, Woods Hole, Massachusetts 02543, U.S.A. ABSTRACT The Aplacophora are shell-less, vermiform mollusks found from the continental shelf regions of the world to depths of 9,000 m. They are grouped into a single class with 2 subclasses: Chaetodermomorpha (= Caudofoveata) and Neomeniomorpha (= Ventroplicida; Solenogastres sensu Salvini-Plawen), a classification which preserves early nomenclature based on Chaetoderma Lovën, validated by the International Commission on Zoological Nomencla-ture. The name "solenogaster" is reserved as a common noun like "clam" or "snail." The Aplacophora have several typical molluscan characters, namely a radula with associated buccal structures; a style sac and mucoid style; a coelom restricted to gonads, a dorsal pericardium, and kidneys; a heart consisting of a ventricle and 2 auricles; laterodorsal-ventral musculature; ventral musculature that bends the body and contained organs dorsally; a dorsal gut; mantle cavity and gills; a vestigial foot which secretes a slime trail; a nervous system of paired ganglionated cords, ladderlike, with pharyngeal ring and buccal ganglia; and finally a development which includes spiral cleavage and a protobranch-like larva. Most of these molluscan characters are not structurally like those of chitons with which they have often been classified; therefore, the Aplacophora are classified separately from the Polyplaco-phora. The 2 aplacophoran taxa resemble each other in their nervous system, coelom, haemocoele, musculature, and shape. Differences in integument between the 2 taxa may be due to reduction in the burrowing Chaetodermomorpha, and in the digestive system due to the obligate coelenterate feeding in the Neomeniomorpha. It is of phylogenetic importance that several characteristic molluscan structures have evolved in the Aplacophora independent of a shell. INTRODUCTION The Aplacophora are worm-shaped mol-lusks surrounded by a cuticle bearing cal-careous spicules; they inhabit the deep ocean basins and continental shelf and slope regions of the world. Observations from more than 380 collections made by the Woods Hole Océanographie Institution, Oregon State University, Scripps Institution of Oceanography, and Centre National de Tri d'Océanographie Biologique show that their greatest species radiation has been in the deep sea. They burrow into or creep on mud; some wrap themselves around alcy-onarians upon which they feed. They are common in the deep sea, occurring in nearly all small-meshed epibenthic dredge hauls (Hessler & Sanders, 1967) and box cores taken from all depths to 9,000 m. In recent years the number of biological sur-veys of the deep sea has increased, and the discovery of numerous new aplacophoran species has rekindled interest in this group. Although most species are still to be des-cribed, some recent taxonomic works are those of Salvini-Plawen (1972, biblio-graphy, for his papers), SchwabI (1963) and Scheltema (1976). CLASSIFICATION The 2 classifications given in Table 1 are currently in use for living mollusks. Both classifications retain the 2 distinct taxa that have been recognized since the 19th cen-tury: one taxon (Neomeniomorpha = Ventroplicida = Solenogastres sensu Salvini-Plawen) is distinguished by a ventral groove containing a narrow foot, the other (Chae-todermomorpha = Caudofoveata) lacks a ventral groove and has an oral shield and a pair of ctenidia. It should be stressed that there has been no modification of member-ship in the 2 aplacophoran taxa in either of 1 Contribution No. 3763 from the Woods Hole Océanographie Institution. (99)
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