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Recent molecular phylogenetic investigations strongly supported the placement of the shell-less, worm-shaped aplacophoran molluscs (Solenogastres and Caudofoveata) and chitons (Polyplacophora) in a clade called Aculifera, which is the sister taxon of all other molluscs. Thus, understanding the evolutionary history of aculiferan molluscs is important for understanding early molluscan evolution. In particular, fundamental questions about evolutionary relationships within Aplacophora have long been unanswered. Here, we supplemented the paucity of available data with transcriptomes from 25 aculiferans and conducted phylogenomic analyses on datasets with up to 525 genes and 75 914 amino acid positions. Our results indicate that aplacophoran taxonomy requires revision as several traditionally recognized groups are non-monophyletic. Most notably, Cavibelonia, the solenogaster taxon defined by hollow sclerites, is polyphyletic, suggesting parallel evolution of hollow sclerites in multiple lineages. Moreover, we describe Apodomenia enigmatica sp. nov. , a bizarre new species that appears to be a morphological intermediate between Solenogastres and Caudofoveata. This animal is not a missing link, however; molecular and morphological studies show that it is a derived solenogaster that lacks a foot, mantle cavity and radula. Taken together, these results shed light on the evolutionary history of Aplacophora and reveal a surprising degree of morphological plasticity within the group.

Aplacophorans have long been argued to be basal molluscs. We present a molecular phylogeny, including the aplacophorans Neomeniomorpha (Solenogastres) and Chaetodermomorpha (Caudofoveata), which recovered instead the clade Aculifera (Aplacophora + Polyplacophora). Our relaxed Bayesian molecular clock estimates an Early Ordovician appearance of the aculiferan crown group consistent with the presence of chiton-like molluscs with seven or eight dorsal shell plates by the Late Cambrian (approx. 501–490 Ma). Molecular, embryological and palaeontological data indicate that aplacophorans, as well as chitons, evolved from a paraphyletic assemblage of chiton-like ancestors. The recovery of cephalopods as a sister group to aculiferans suggests that the plesiomorphic condition in molluscs might be a morphology similar to that found in monoplacophorans.

The position of the earliest-derived living molluscs, the Polyplacophora (chitons) and shell-less vermiform Aplacophora, remains highly contentious despite many morphological, developmental and molecular studies of extant organisms. These two groups are thought to represent either a basal molluscan grade or a clade (Aculifera) sister to the ‘higher’ molluscs (Conchifera). These incompatible hypotheses result in very different predictions about the earliest molluscs. A new cladistic analysis incorporating both Palaeozoic and extant molluscs is presented here. Our results support the monophyly of Aculifera and suggest that extant aplacophorans and polyplacophorans both derive from a disparate group of multivalved molluscs in two major clades. Reanalysis of the critical Ordovician taxon ‘Helminthochiton’ thraivensis shows that this animal lacks a true foot despite bearing polyplacophoran-like valves. Its position within our phylogenetic reconstruction indicates that many fossil ‘polyplacophorans’ in the order Palaeoloricata are likely to represent footless stem-group aplacophorans. ‘H.’ thraivensis and similar forms such as Acaenoplax may be morphological stepping stones between chitons and the shell-less aplacophorans. Our results imply that crown-group molluscan synapomorphies include serial repetition, the presence of a foot, a mineralized scleritome and a creeping rather than worm-like mode of life.

Articulated halkieriids ofHalkieria evangelistasp. nov. are described from the Sirius Passet fauna in the Lower Cambrian Buen Formation of Peary Land, North Greenland. Three zones of sclerites are recognizable: obliquely inclined rows of dorsal palmates, quincuncially inserted lateral cultrates and imbricated bundles of ventro-lateral siculates. In addition there is a prominent shell at both ends, each with radial ornamentation. Both sclerites and shells were probably calcareous, but increase in body size led to insertion of additional sclerites but marginal accretion of the shells. The ventral sole was soft and, in life, presumably muscular. Recognizable features of internal anatomy include a gut trace and possible musculature, inferred from imprints on the interior of the anterior shell. Halkieriids are closely related to the Middle CambrianWixaxia, best known from the Burgess Shale: this clade appears to have played an important role in early protostome evolution. From an animal fairly closely related toWixaxiaarose the polychaete annelids; the bundles of siculate sclerites prefigure the neurochaetae whereas the dorsal notochaetae derive from the palmates.Wixaxiaappears to have a relic shell and a similar structure in the sternaspid polychaetes may be an evolutionary remnant. The primitive state in extant polychaetes is best expressed in groups such as chrysopetalids, aphroditaceans and amphinomids. The homology between polychaete chaetae and the mantle setae of brachiopods is one line of evidence to suggest that the latter phylum arose from a juvenile halkieriid in which the posterior shell was first in juxtaposition to the anterior and rotated beneath it to provide the bivalved condition of an ancestral brachiopod.H. evangelistasp. nov. has shells which resemble those of a brachiopod; in particular the posterior one. From predecessors of the halkieriids known as siphogonuchitids it is possible that both chitons (polyplacophorans) and conchiferan molluscs arose. The hypothesis of halkieriids and their relatives having a key role in annelid—brachiopod—mollusc evolution is in accord with some earlier proposals and recent evidence from molecular biology. It casts doubt, however, on a number of favoured concepts including the primitive annelid being oligochaetoid and a burrower, the brachiopods being deuterostomes and the coelom being an archaic feature of metazoans. Rather, the annelid coelom arose as a functional consequence of the transition from a creeping halkieriid to a polychaete with stepping parapodial locomotion.

By virtue of the structure of the shell, the morphology of the heart and gills and the characteristics of the larval form, the Bivalvia are sharply distinct from other Conchifera. The closest allies of the Bivalvia are the Polyplacophora and Aplacophora, and these three should be united in the subphylum Peltophora, the remaining molluscan classes being contained in the Cochleophora. After the first phylogenetic appearance of the Bivalvia (close to the most primitive Polyplacophora), the following stages ensued: (1) formation of a hypothetical monomyarian form with a straight dorsal margin; (2) formation of the Protobranchia; (3) formation of the Autobranchia, with ciliary water-transport and suspension-filtering gills; (4) formation of the Septibranchia, with a pallial septum forming a membrane pump; (5) adaptive radiation of the three above mentioned superorders. Arguments are considered in favour of the opinion that the Rostroconchia constitute an order of the Septibranchia rather than a separate class. A proposed scheme of the phylogeny and systematics of the Bivalvia is presented.

A detailed redescription of the Middle Cambrian metazoan Wiwaxia corrugata (Matthew) is given with the aid of photographs, mostly taken under ultraviolet radiation, and explanatory camera lucida drawings. Wiwaxia is known only from the Stephen Formation with four localities within the Bathyuriscus-Elrathina Zone, the celebrated Burgess Shale (Phyllopod bed) and Ogygopsis Shale and two localities that appear to expose strata relatively high in the Formation, and a new locality in the underlying Glossopleura Zone. Specimens from the Ogygopsis Shale consist almost entirely of isolated sclerites, whereas in the Phyllopod bed the species is also known from entire specimens, semi-isolated assemblages and isolated soft-parts. T he description here is based almost entirely on Phyllopod bed m aterial. Wiwaxia was originally largely covered by sclerites. On the basis of shape and arrangement five distinct zones are recognizable: dorsal, upper lateral, lower lateral, anterior and ventro-lateral. In addition, there was a row of elongate dorso-lateral spinose sclerites running along either side of the body. Each sclerite was inserted separately via a root-like structure. Growth of the animal appears to have occurred by moulting of the sclerites; one unique juvenile specimen appears to be preserved in the act of moulting. The ventral surface was apparently an area of soft tissue devoid of sclerites. Little is known of the internal anatomy, although anteriorly there was a feeding apparatus consisting of two rows of posteriorly directed teeth. Most stages of growth are known varying from presumed juveniles, which at their smallest may have lacked spines, to adults over 50 mm long. Wiwaxia appears to have been an epifaunal deposit feeder that crawled across the sea-bed, although smaller juveniles may have been infaunal. The dorso-lateral spines may have provided protection against predators and the existence of broken spines suggests the successful deterrence of attack. The affinities of Wiwaxia do not appear to lie with the polychaetous annelids. While the possibilities of convergence cannot be eliminated, there appears to be a significant similarity between Wiwaxia and molluscs that suggests a related derivation from a turbellarian ancestor. Nevertheless, Wiwaxia has a distinctive bodyplan and as such cannot be accommodated in any known phylum. While Wiwaxia is unique to the Stephen Formation isolated sclerites from Lower Cambrian rocks appear to represent earlier wiwaxiids, although these sclerites show differences in their size and composition as com pared with Wiwaxia . Wiwaxia enhances knowledge of early metazoan evolution and ecology in the Cambrian. In particular, it gives fresh insights into the origin and relative success of certain metazoan bodyplans, the origin of trace fossils, and the importance of predation in Cambrian communities.

The chance finding of a single adult specimen of the solenogastre Neomenia carinata Tullberg 1875 rendered possible an embryological study of this species. Little is known concerning the ontogeny of the Aplacophora and a number of important questions, such as the fate of the larval test, the nature of the abapical depression visible during gastrulation and the presence or absence of any evidence of metamery, remain to be elucidated. Embryos, larvae and post-larvae were maintained in laboratory culture a t 10 °C. No description can be given of the early cleavage stages since the eggs when found were always well advanced. Each egg is enclosed by a single membrane. Gastrulation begins on the second day, by a process of immigration of the abapical cells; the abapical depression, often called a blastopore, is shown to be of an unusual character and is to be referred to as a pseudo -blastopore. This pseudo-blastopore is merely a relatively shallow depression marking the area at which immigration is occurring. After the completion of gastrulation, the cells lining the pseudo-blastopore are the prospective trunk ectoderm. The endoderm and mesoderm lie within the embryo and have no communication with the exterior. The remaining cells form the larval test, except for an apical quartet which will develop into the larval apical plate and for six small patches of cells which will give rise to much of the definitive nervous system. The apical/abapical axis of the gastrula is coincident with the antero-posterior axis of the adult solenogastre. The embryos leave their egg membranes on the third day and swim by means of the cilia of the larval test. This test becomes organized into a series of tiers of regularly shaped cells. The main tier is the prototroch, on which is developed a strong equatorial band of locomotory cilia. The larvae are not negatively geotactic and swim close to the bottom of a culture vessel. Proliferation of the definitive nervous tissue begins just before hatching, from six areas of larval test ectoderm on the future ventral side. Nervous elements are cut off inwards at the bases of shallow ectodermal depressions; they come to aggregate into cerebral and ventral (pedal) ganglia. By the seventh day the rudiment of the adult trunk is visible, protruding through the pseudo-blastopore. On its tip is the yolk-laden, ciliated, larval telotroch. The remainder of the trunk is 'Unciliated (except for a median longitudinal ventral ciliated band) but bears numerous pointed calcareous spicules. The length of the trunk rudiment increases by repeated division of the ectodermal cells within the pseudo-blastopore. The midgut passes down into the trunk and with it travel mesodermal elements and a pair of bands of nervous tissue which will form the ventral (pedal) cords. Proctodaeal and stomodaeal invaginations place the midgut in communication with the exterior but the larvae do not feed. The ‘ pygidial ’ development of the trunk of Neomenia resembles strongly that process as found in many annelids but it must be noted that no trace of metameric segmentation of this trunk is visible at any stage in the development of Neomenia . At no stage does the trunk bear overlapping dorsal spicules like those described by Pruvot for Nematomenia ; it seems probable on embryological grounds that the solenogastres are more closely allied to the primitive Lamellibranchia than to the Polyplacophora. Metamorphosis is considered to include only those changes occurring from the tenth to the thirteenth days, during which period the larva exchanges a pelagic for a benthic life. The trunk comes to form by far the greater proportion of the late larva and swimming becomes impossible. The larval test cells lose their orderly arrangement, the prototroch ceases to exist as a co-ordinated locomotory organ and the whole larval test becomes enclosed within the blastocoel of the trunk by the anterior extension and fusion of folds of definitive ectoderm. Similarly, the larval telotroch enters the trunk blastocoel posteriorly. From the blastocoel these yolk-laden cells of the larval locomotory and sensory apparatus pass through the midgut wall into the digestive cells; here they are broken down into small clusters of yolk granules which form the main identifiable food reserve of the post-larva. The mouth and anus, which, before metamorphosis, were directed posteriorly, are now directed ventrally; they lie at the anterior and posterior extremities of a median ventral longitudinal ciliated groove, the so-called pedal groove of the post-larva. This groove is at no stage employed as a pedal sole. The sites from which nervous elements were proliferated during larval life are obliterated at metamorphosis. In the post-larva, two new pairs of ectodermal nervous depressions develop. Both give rise to tubular strands of nervous tissue which extend to and fuse with the cerebral ganglia. Lateral (pleural) cords develop as outgrowths from the cerebral ganglia. Post-larval stages lived in the laboratory without food for up to 10 weeks; they were subsisting entirely on their food reserves. The natural diet of the species is unknown. During the ninth week after metamorphosis the atrium appeared, as a capacious invagination around the mouth. No radula, gills, cloaca, heart, coelomoducts or gonads developed before the young stages died; all but the first are known to be present in the adult Neomenia . A bibliography of works dealing with the ontogeny of Aplacophora is given.