172 Structure and Evolution of Invertebrate Nervous Systems Edited by Andreas Schmidt-Rhaesa, Stefen Harzsch, and Günter Purschke © Oxford University Press 2016. Published 2016 by Oxford University Press. INTRODUCTION Molluscs are the second-most speciose metazoan phylum, after Arthropoda, and arguably molluscs demonstrate the largest morphological disparity. he dramatic body-plan modiications and changes among the molluscan clades leave few consistent characters that can be directly compared across all eight living classes 1 . he living diversity encompasses bivalves (clams, mus- sels, and shipworms), gastropods (snails and slugs), and cephalo- pods (squid and octopuses), each of which are treated in separate chapters in this volume, and ive ‘minor’ or more precisely less speciose classes that we discuss in this chapter (Fig. 18.1). he ive groups covered here—Caudofoveata (chaetoderms), Monoplacophora (headless deep-sea limpets), Polyplacophora (chitons), Scaphopoda (tusk shells), and Solenogastres (neome- niomorphs)—are addressed in alphabetical order in a deliberate efort to treat each group on its own evidence and to avoid sug- gestion of any speciic phylogenetic hypotheses. hese animals are all exclusively marine, and live as benthic or infaunal species. hey are less commercially exploited than the three major classes, but nonetheless demonstrate extensive diversiication within each clade, and many are locally abundant and exert signiicant ecosys- tem control (Dethier and Duggins 1984, Shimek 1988). Careful consideration of these groups is essential towards resolving larger questions of molluscan, and metazoan, evolutionary dynamics. Molluscs are soft-bodied animals that ancestrally possess a mus- cular locomotory foot, a mineralized proteinaceous tooth structure called the radula, and a calcium carbonate shell which is anchored to the foot by dorsal–ventral muscle bundles. he mantle is a tissue layer that creates a chamber that houses the viscera and secretes the shell, though this is subject to interpretation and modiica- tion especially in these ‘minor’ classes. Chemosensory epithelia called osphradia, apparently unique to molluscs, are present in most members of some classes (Lindberg and Sigwart 2015). All 18 MOLLUSCA: CAUDOFOVEATA, MONOPLACOPHORA, POLYPLACOPHORA, SCAPHOPODA, AND SOLENOGASTRES Julia D. Sigwart and Lauren H. Sumner-Rooney of these structures are lost in one or more of the eight living classes: the vermiform Caudofoveata have no foot, Bivalvia have no radula, at least Scaphopoda, Monoplacophora, Solenogastres, and Caudofoveata lack an osphradium, Caudofoveata and Solenogastres also lack shells. Extensive shell loss and modiications are seen repeatedly in Gastropoda, Bivalvia, Cephalopoda, and to a lesser extent Polyplacophora. he dynamics of these extreme body plan modiications within a single phylum present a fundamentally important question in evolutionary biology. here is no clear consensus on the topology of phylogenetic relationships within the molluscan classes (Sigwart and Lindberg 2015). he challenging problem of resolving the sister relationships of such variable forms has perhaps been further hampered by a historical focus on shell forms, which are evidently extremely plastic. Two alternative hypotheses are both supported by molecular and morphological evidence, and the diferences between these two topologies provide little clarity on the sister rela- tionships between most clades. he ‘Aculifera’ hypothesis unites the vermiform aplacophorans (Caudofoveata and Solenogastres) and the eight-valved Polyplacophora, in a clade opposed to ‘Conchifera’, containing the other ive classes. his topology is supported by the only phylogenomic analysis of total-group Mollusca to date (Smith et al. 2011), and fossils of ‘armoured apla- cophorans’, sharing characters of both chitons and spiculose ver- miform molluscs, have been described as transitional aculiferans (Sigwart & Sutton 2007, Sutton et al. 2012, Sutton and Sigwart 2012). he almost completely contradictory ‘Serialia’ hypothesis was originally proposed from the irst molecular genetic sequence recovered from a rare living monoplacophoran, and has been sub- sequently recovered by independent analyses with multiple mono- placophoran species (Giribet et al. 2006, Kano et al. 2012, Stöger et al. 2013). he topology associated with Serialia is supported by strong anatomical arguments and fossil stratigraphic analysis (Stöger et al. 2013). Very few studies have included all eight classes (Giribet et al. 2006, Smith et al. 2011, Stöger et al. 2013), and more sources of independent evidence, representing all groups, are required to resolve molluscan relationships. 1 Each of these clades, with equivalent Linnean rank as a class, represents a mono- phyletic group hierarchically subordinate to the total-group Mollusca.