Hox genes in sea spiders (Pycnogonida) and the homology of arthropod head segments

The pycnogonids (or sea spiders) are an enigmatic group of arthropods, classified in recent phylogenies as a sister-group of either euchelicerates (horseshoe crabs and arachnids), or all other extant arthropods. Because of their bizarre morpho-anatomy, homologies with other arthropod taxa have been difficult to assess. We review the main morphology-based hypotheses of correspondence between anterior segments of pycnogonids, arachnids and mandibulates. In an attempt to provide new relevant data to these controversial issues, we performed a PCR survey of Hox genes in two pycnogonid species, Endeis spinosa and Nymphon gracile, from which we could recover nine and six Hox genes, respectively. Phylogenetic analyses allowed to identify their orthology relationships. The Deformed gene from E. spinosa and the abdominal-A gene from N. gracile exhibit unusual sequence divergence in their homeodomains, which, in the latter case, may be correlated with the extreme reduction of the posterior region in pycnogonids. Expression patterns of two Hox genes (labial and Deformed) in the E. spinosa protonymphon larva are discussed. The anterior boundaries of their expression domains favour homology between sea spider chelifores, euchelicerates chelicerae and mandibulate (first) antennae, in contradistinction with previously proposed alternative schemes such as the protocerebral identity of sea spider chelifores or the absence of a deutocerebrum in chelicerates. In addition, while anatomical and embryological evidences suggest the possibility that the ovigers of sea spiders could be a duplicated pair of pedipalps, the Hox data support them as modified anterior walking legs, consistent with the classical views.Supplementary material is available for this article at and is accessible for authorized users.Guest editors Jean Deutsch and Gerhard Scholtz     

Wiring a periscope--ocelli, retinula axons, visual neuropils and the ancestrality of sea spiders

The Pycnogonida or sea spiders are cryptic, eight-legged arthropods with four median ocelli in a ‘periscope’ or eye tubercle. In older attempts at reconstructing phylogeny they were Arthropoda incertae sedis, but recent molecular trees placed them as the sister group either to all other euchelicerates or even to all euarthropods. Thus, pycnogonids are among the oldest extant arthropods and hold a key position for the understanding of arthropod evolution. This has stimulated studies of new sets of characters conductive to cladistic analyses, e.g. of the chelifores and of the hox gene expression pattern. In contrast knowledge of the architecture of the visual system is cursory. A few studies have analysed the ocelli and the uncommon “pseudoinverted” retinula cells. Moreover, analyses of visual neuropils are still at the stage of Hanström''s early comprehensive works. We have therefore used various techniques to analyse the visual fibre pathways and the structure of their interrelated neuropils in several species. We found that pycnogonid ocelli are innervated to first and second visual neuropils in close vicinity to an unpaired midline neuropil, i.e. possibly the arcuate body, in a way very similar to ancestral euarthropods like Euperipatoides rowelli (Onychophora) and Limulus polyphemus (Xiphosura). This supports the ancestrality of pycnogonids and sheds light on what eyes in the pycnogonid ground plan might have ‘looked’ like. Recently it was suggested that arthropod eyes originated from simple ocelli similar to larval eyes. Hence, pycnogonid eyes would be one of the early offshoots among the wealth of more sophisticated arthropod eyes.     

Cephalic and Limb Anatomy of a New Isoxyid from the Burgess Shale and the Role of “Stem Bivalved Arthropods” in the Disparity of the Frontalmost Appendage

We herein describe Surusicaris elegans gen. et sp. nov. (in Isoxyidae, amended), a middle (Series 3, Stage 5) Cambrian bivalved arthropod from the new Burgess Shale deposit of Marble Canyon (Kootenay National Park, British Columbia). Surusicaris exhibits 12 simple, partly undivided biramous trunk limbs with long tripartite caeca, which may illustrate a plesiomorphic “fused” condition of exopod and endopod. We construe also that the head is made of five somites (= four segments), including two eyes, one pair of anomalocaridid-like frontalmost appendages, and three pairs of poorly sclerotized uniramous limbs. This fossil may therefore be a candidate for illustrating the origin of the plesiomorphic head condition in euarthropods, and questions the significance of the “two-segmented head” in, e.g., fuxianhuiids. The frontalmost appendage in isoxyids is intriguingly disparate, bearing similarities with both dinocaridids and euarthropods. In order to evaluate the relative importance of bivalved arthropods, such as Surusicaris, in the hypothetical structuro-functional transition between the dinocaridid frontal appendage and the pre-oral—arguably deutocerebral—appendage of euarthropods, we chose a phenetic approach and computed morphospace occupancy for the frontalmost appendages of 36 stem and crown taxa. Results show different levels of evolutionary decoupling between frontalmost appendage disparity and body plans. Variance is greatest in dinocaridids and “stem bivalved” arthropods, but these groups do not occupy the morphospace homogeneously. Rather, the diversity of frontalmost appendages in “stem bivalved” arthropods, distinct in its absence of clear clustering, is found to link the morphologies of “short great appendages,” chelicerae and antennules. This find fits the hypothesis of an increase in disparity of the deutocerebral appendage prior to its diversification in euarthropods, and possibly corresponds to its original time of development. The analysis of this pattern, however, is sensitive to the—still unclear—extent of polyphyly of the “stem bivalved” taxa.     

Transitions in functional morphology from “large branchiopods” to Cladocera: Video and confocal microscopic studies of Cyclestheria hislopi (Cyclestherida) and Sida crystallina (Cladocera: Ctenopoda)

Great diversity is found in morphology and functionality of arthropod appendages, both along the body axis of individual animals and between different life-cycle stages. Despite many branchiopod crustaceans being well known for displaying a relatively simple arrangement of many serially post-maxillary appendages (trunk limbs), this taxon also shows an often unappreciated large variation in appendage morphology. Diplostracan branchiopods exhibit generally a division of labor into locomotory antennae and feeding/filtratory post-maxillary appendages (trunk limbs). We here study the functionality and morphology of the swimming antennae and feeding appendages in clam shrimps and cladocerans and analyze the findings in an evolutionary context (e.g., possible progenetic origin of Cladocera). We focus on Cyclestheria hislopi (Cyclestherida), sister species to Cladocera and exhibiting many “large” branchiopod characters (e.g., many serially similar appendages), and Sida crystallina (Cladocera, Ctenopoda), which likely exhibits plesiomorphic cladoceran traits (e.g., six pairs of serially similar appendages). We combine (semi-)high-speed recordings of behavior with confocal laser scanning microscopy analyses of musculature to infer functionality and homologies of locomotory and filtratory appendages in the two groups. Our morphological study shows that the musculature in all trunk limbs (irrespective of limb size) of both C. hislopi and S. crystallina comprises overall similar muscle groups in largely corresponding arrangements. Some differences between C. hislopi and S. crystallina, such as fewer trunk limbs and antennal segments in the latter, may reflect a progenetic origin of Cladocera. Other differences seem related to the appearance of a specialized type of swimming and feeding in Cladocera, where the anterior locomotory system (antennae) and the posterior feeding system (trunk limbs) have become fully separated functionally from each other. This separation is likely one explanation for the omnipresence of cladocerans, which have conquered both freshwater and marine free water masses and a number of other habitats.     

Co-option of an anteroposterior head axis patterning system for proximodistal patterning of appendages in early bilaterian evolution

The enormous diversity of extant animal forms is a testament to the power of evolution, and much of this diversity has been achieved through the emergence of novel morphological traits. The origin of novel morphological traits is an extremely important issue in biology, and a frequent source of this novelty is co-option of pre-existing genetic systems for new purposes (Carroll et al., 2008). Appendages, such as limbs, fins and antennae, are structures common to many animal body plans which must have arisen at least once, and probably multiple times, in lineages which lacked appendages. We provide evidence that appendage proximodistal patterning genes are expressed in similar registers in the anterior embryonic neurectoderm of Drosophila melanogaster and Saccoglossus kowalevskii (a hemichordate). These results, in concert with existing expression data from a variety of other animals suggest that a pre-existing genetic system for anteroposterior head patterning was co-opted for patterning of the proximodistal axis of appendages of bilaterian animals.     

Anatomical network analysis of the musculoskeletal system reveals integration loss and parcellation boost during the fins‐to‐limbs transition

Tetrapods evolved from within the lobe‐finned fishes around 370 Ma. The evolution of limbs from lobe‐fins entailed a major reorganization of the skeletal and muscular anatomy of appendages in early tetrapods. Concurrently, a degree of similarity between pectoral and pelvic appendages also evolved. Here, we compared the anatomy of appendages in extant lobe‐finned fishes (Latimeria and Neoceratodus) and anatomically plesiomorphic amphibians (Ambystoma, Salamandra) and amniotes (Sphenodon) to trace and reconstruct the musculoskeletal changes that took place during the fins‐to‐limbs transition. We quantified the anatomy of appendages using network analysis. First, we built network models—in which nodes represent bones and muscles, and links represent their anatomical connections—and then we measured network parameters related to their anatomical integration, heterogeneity, and modularity. Our results reveal an evolutionary transition toward less integrated, more modular appendages. We interpret this transition as a diversification of muscle functions in tetrapods compared to lobe‐finned fishes. Limbs and lobe‐fins show also a greater similarity between their pectoral and pelvic appendages than ray‐fins do. These findings on extant species provide a basis for future quantitative and comprehensive reconstructions of the anatomy of limbs in early tetrapod fossils, and a way to better understand the fins‐to‐limbs transition.     

Appendage-restricted gene induction using a heated agarose gel for studying regeneration in metamorphosed Xenopus laevis and Pleurodeles waltl

Amphibians and fish often regenerate lost parts of their appendages (tail, limb, and fin) after amputation. Limb regeneration in adult amphibians provides an excellent model for appendage (limb) regeneration through 3D morphogenesis along the proximodistal, dorsoventral, and anteroposterior axes in mammals, because the limb is a homologous organ among amphibians and mammals. However, manipulating gene expression in specific appendages of adult amphibians remains difficult; this in turn hinders elucidation of the molecular mechanisms underlying appendage regeneration. To address this problem, we devised a system for appendage-specific gene induction using a simplified protocol named the “agarose-embedded heat shock (AeHS) method” involving the combination of a heat-shock-inducible system and insertion of an appendage in a temperature-controlled agarose gel. Gene expression was then induced specifically and ubiquitously in the regenerating limbs of metamorphosed amphibians, including a frog (Xenopus laevis) and newt (Pleurodeles waltl). We also induced gene expression in the regenerating tail of a metamorphosed P. waltl newt using the same method. This method can be applied to adult amphibians with large body sizes. Furthermore, this method enables simultaneous induction of gene expression in multiple individuals; further, the data are obtained in a reproducible manner, enabling the analysis of gene functions in limb and tail regeneration. Therefore, this method will facilitate elucidation of the molecular mechanisms underlying appendage regeneration in amphibians, which can support the development of regenerative therapies for organs, such as the limbs and spinal cord.     

Morphogenesis of Pseudopallene sp. (Pycnogonida, Callipallenidae) I: embryonic development

Embryonic development of Pycnogonida (sea spiders) is poorly understood in comparison to other euarthropod lineages with well-established model organisms. However, given that pycnogonids potentially represent the sister group to chelicerates or even to all other euarthropods, their development might yield important data for the reconstruction of arthropod evolution. Using scanning electron microscopy, fluorescent nucleic staining and immunohistochemistry, the general course of embryonic morphogenesis in Pseudopallene sp. (Callipallenidae), a pycnogonid with prolonged embryonic development, is described. A staging system comprising ten stages is presented, which can be used in future studies addressing specific developmental processes. The initially slit-like stomodeum anlage forms at the anterior end of an eight-shaped germ band and predates proboscis outgrowth. The latter process is characterized by the protrusion of three cell populations that are subsequently involved in pharynx formation. In later stages, the proboscis assumes distally a horseshoe-like shape. At no time, a structure corresponding to the euarthropod labrum is detectable. Based on the complete lack of palpal and ovigeral embryonic limbs and the early differentiation of walking leg segments 1 and 2, the existence of an embryonized protonymphon stage during callipallenid development is rejected. The evolution of pycnogonid hatching stages, especially within Callipallenidae and Nymphonidae, is re-evaluated in the light of recent phylogenetic analyses. Specifically, the re-emergence of the ancestral protonymphon larva (including re-development of palpal and ovigeral larval limbs) and a possible re-appearance of adult palps in the nymphonid lineage are discussed. This challenges the perception of pycnogonid head appendage evolution as being driven by reduction events alone.     

Evolution of the chelicera: a dachshund domain is retained in the deutocerebral appendage of Opiliones (Arthropoda,Chelicerata)

The proximo‐distal axis of the arthropod leg is patterned by mutually antagonistic developmental expression domains of the genes extradenticle, homothorax, dachshund, and Distal‐less. In the deutocerebral appendages (the antennae) of insects and crustaceans, the expression domain of dachshund is frequently either absent or, if present, is not required to pattern medial segments. By contrast, the dachshund domain is entirely absent in the deutocerebral appendages of spiders, the chelicerae. It is unknown whether absence of dachshund expression in the spider chelicera is associated with the two‐segmented morphology of this appendage, or whether all chelicerates lack the dachshund domain in their chelicerae. We investigated gene expression in the harvestman Phalangium opilio, which bears the plesiomorphic three‐segmented chelicera observed in “primitive” chelicerate orders. Consistent with patterns reported in spiders, in the harvestman chelicera homothorax, extradenticle, and Distal‐less have broadly overlapping developmental domains, in contrast with mutually exclusive domains in the legs and pedipalps. However, unlike in spiders, the harvestman chelicera bears a distinct expression domain of dachshund in the proximal segment, the podomere that is putatively lost in derived arachnids. These data suggest that a tripartite proximo‐distal domain structure is ancestral to all arthropod appendages, including deutocerebral appendages. As a corollary, these data also provide an intriguing putative genetic mechanism for the diversity of arachnid chelicerae: loss of developmental domains along the proximo‐distal axis.     

The Sp8 zinc-finger transcription factor is involved in allometric growth of the limbs in the beetle Tribolium castaneum

Members of the Sp gene family are involved in a variety of developmental processes in both vertebrates and invertebrates. We identified the ortholog of the Drosophila Sp-1 gene in the red flour beetle Tribolium castaneum, termed T-Sp8 because of its close phylogenetic relationship to the vertebrate Sp8 genes. During early embryogenesis, T-Sp8 is seen in segmental stripes. During later stages, TSp8 is dynamically expressed in the limb buds of the Tribolium embryo. At the beginning of bud formation, TSp8 is uniformly expressed in all body appendages. As the limbs elongate, a ring pattern develops sequentially and the expression profile at the end of embryogenesis correlates with the final length of the appendage. In limbs that do not grow out like the labrum and the labium, T-Sp8 expression remains uniform, whereas a two-ring pattern develops in the longer antennae and the maxillae. In the legs that elongate even further, four rings of T-Sp8 expression can be seen at the end of leg development. The role of T-Sp8 for appendage development was tested using RNAi. Upon injection of double stranded T-Sp8 RNA, larvae develop with dwarfed appendages. Affected T-Sp8(RNAi) legs were tested for the presence of medial and distal positional values using the expression marker genes dachshund and Distal-less, respectively. The results show that a dwarfed TSp8(RNAi) leg consists of proximal, medial and distal parts and argues against T-Sp8 being a leg gap gene. Based on the differential expression pattern of T-Sp8 in the appendages of the head and the thorax and the RNAi phenotype, we hypothesise that T-Sp8 is involved in the regulation of limb-length in relation to body size - a process called allometric growth.     

A Larval Sea Spider (Arthropoda: Pycnogonida) from the Upper Cambrian 'orsten' of Sweden, and the Phylogenetic Position of Pycnogonids

Among a set of small, secondarily phosphatised larval arthropods from the Upper Cambrian 'Orsten' of Sweden, described by Müller and Walossek in 1986, one form bears a remarkable resemblance to the hatching protonymph larva of extant Pantopoda. This 'larva D' shares with protonymphs their gross body form, the anteroventral mouth on a slightly off-set forehead region, the cheliceral morphology, two homeomorphic pairs of post-cheliceral limbs, and further detailed similarities. It is described herein as Cambropycnogon klausmuelleri gen. et sp. nov. and is proposed as the oldest unequivocal record of both Pycnogonida and Chelicerata. Plesiomorphic features such as a pair of rudimentary pre-cheliceral limbs and the gnathobasic basipods of the two post-cheliceral limbs distinguish it from all known larvae of extant Pantopoda and lead us to propose a phylogeny of the Pycnogonida of the form ( Cambropycnogon klausmuelleri + ( Palaeoisopus + ( Palaeopantopus + Pantopoda))). The fossil may help to resolve the long debate about the relationships of Pycnogonida to other Arthropoda and supports a (Pycnogonida + Euchelicerata) relationship within the Chelicerata. The pre-cheliceral limbs in this fossil support traditional morphological studies in which the chelicera represent the second (a2) head appendage, corresponding to the crustacean 'second antennae', and contradict recent data based on homeobox genes implying that the chelicerae are the first (a1) head appendages homologous with crustacean first antennae.     

Morphogenesis of Pseudopallene sp. (Pycnogonida, Callipallenidae) II: postembryonic development

Pycnogonida (sea spiders) are bizarre marine arthropods that are nowadays most frequently considered as being the sister group to all other chelicerates. The majority of pycnogonid species develops via a protonymphon larva with only three pairs of limbs affiliated with the future head region. Deviating from this, the hatching stage of some representatives shows already an advanced degree of trunk differentiation. Using scanning electron microscopy, fluorescent nucleic staining, and bright-field stereomicroscopy, postembryonic development of Pseudopallene sp. (Callipallenidae), a pycnogonid with an advanced hatching stage, is described. Based on external morphology, six postembryonic stages plus a sub-adult stage are distinguished. The hatching larva is lecithotrophic and bears the chelifores as only functional appendage pair and unarticulated limb buds of walking leg pairs 1 and 2. Palpal and ovigeral larval limbs are absent. Differentiation of walking leg pairs 3 and 4 is sequential. Apart from the first pair, each walking leg goes through a characteristic sequence of three externally distinct stages with two intermittent molts (limb bud-seven podomeres-nine podomeres). First external signs of oviger development are detectable in postembryonic stage 3 bearing three articulated walking leg pairs. Following three more molts, the oviger has attained adult podomere composition. The advanced hatching stages of different callipallenids are compared and the inclusive term "walking leg-bearing larva" is suggested, as opposed to the behavior-based name "attaching larva". Data on temporal and structural patterns of walking leg differentiation in other pycnogonids are reviewed and discussed. To facilitate comparisons of walking leg differentiation patterns across many species, we propose a concise notation in matrix fashion. Due to deviating structural patterns of oviger differentiation in another callipallenid species as well as within other pycnogonid taxa, evolutionary conservation of characteristic stages of oviger development is not apparent even in closely related species.     

A scanning electron microscopy study of the embryonic development of Pycnogonum litorale (Arthropoda,Pycnogonida)

The phylogenetic position of the enigmatic Pycnogonida (sea spiders) is still controversial. This is in part due to a lack of detailed data about the morphology and ontogenesis of this, in many aspects, aberrant group. In particular, studies on the embryonic development of pycnogonids are rare and in part contradictory. Here, we present the first embryological study of a pycnogonid species using scanning electron microscopy (SEM). We describe the late embryogenesis of Pycnogonum litorale from the first visible appendage anlagen to the hatchling in 11 embryonic stages. The three pairs of appendage anlagen gain in length by growth, as well as by extension of furrows into the embryo. The opening of the stomodaeum is located far in front of the anlagen of the chelifores and has a Y‐shaped lumen from the onset. During further embryogenesis, the position of the mouth shifts ventrally, until it is located between the chelifores. The proboscis anlage grows out as a circumoral wall‐like structure, which is initially more pronounced ventrally. Hypotheses about the evolution of the proboscis by fusion of originally separated components are critically discussed, because the proboscis anlage of P. litorale shows no indications of a composite nature. In particular, a participation of post‐cheliforal elements in proboscis formation is rejected by our data. Further, no preoral structure and no stage in proboscis formation was found, which could plausibly be homologized with the labrum of othereuarthropods. Thus, our study supports the assumption of a complete lack of a labrum in Pycnogonida. J. Morphol., 2010. © 2010 Wiley‐Liss, Inc.     

A new 'great-appendage' arthropod from the Lower Cambrian of China and homology of chelicerate chelicerae and raptorial antero-ventral appendages

The uniramous ‘great appendages’ of several arthropods from the Early to Middle Cambrian are a characteristic pair of pre‐oral limbs, which served for prey capture. It has been assumed that the morphological differences between the ‘great‐appendage’ arthropods indicate that raptorial antero‐ventral and anteriorly pointing appendages evolved more than once in arthropod phylogeny. One set of Cambrian ‘great‐appendage’ arthropods has, however, very similar short antero‐ventral appendages with a peduncle of two segments angled against each other (elbowed) and with stout distally or medio‐distally directed spines or long flexible flagellate spines on each of the four distal segments. Moreover, the head appendages of all these forms comprise the ‘great appendages’ and three pairs of biramous limbs. To this set of taxa we can add a new form from the Lower Cambrian Maotianshan Shale of southern China, Haikoucaris ercaiensis n. gen. and n. sp. It is known from three specimens, possibly being little abundant in the faunal community. It can be distinguished from all other taxa by the prominence of the proximal claw segment of its ‘great appendages’ and by only three distal spines (one on each of the distal segments). The similarity of the short, spiky ‘great appendages’ of Haikoucaris with the chelicera of the Chelicerata leads us to hypothesize that this particular type of ‘great appendages’ was the actual precursor of the chelicera. Homeobox gene and developmental data recently demonstrated the homology between the antenna of ateloceratans and the antennula of crustaceans on one side and the chelicera of chelicerates on the other. To this we add palaeontological evidence for the homology between the chelicerae of chelicerates and the ‘short great appendages’ of certain Cambrian arthropods, which leads us to hypothesize that the evolutionary path went from the ‘short great appendages’, by progressive compaction, toward the chelicera with only a two‐spined chela. The new form from China is regarded as the possible latest offshoot, whereas the other ‘great appendages’ arthropods with similar short grasping limbs were derivatives of the stem lineage of the crown‐group Chelicerata. Consequently, the chelicera with a chela with one fixed and one mobile finger is an autapomorphy of the crown group of Chelicerata, whereas a raptorial, but more limb‐like antenna, with more distal spine‐bearing segments, characterized the ground pattern of Chelicerata. Further taxa having ‘great appendages’, including the large Anomalocarididae, are also discussed in the light of their possible affinities to the Chelicerata and possible monophyly of all of these arthropods with raptorial anterior appendages.     

Trilobite tagmosis and body patterning from morphological and developmental perspectives

The Trilobita were characterized by a cephalic region in whichthe biomineralized exoskeleton showed relatively high morphologicaldifferentiation among a taxonomically stable set of well definedsegments, and an ontogenetically and taxonomically dynamic trunkregion in which both exoskeletal segments and ventral appendageswere similar in overall form. Ventral appendages were homonomousbiramous limbs throughout both the cephalon and trunk, exceptfor the most anterior appendage pair that was antenniform, preoral,and uniramous, and a posteriormost pair of antenniform cerci,known only in one species. In some clades trunk exoskeletalsegments were divided into two batches. In some, but not all,of these clades the boundary between batches coincided withthe boundary between the thorax and the adult pygidium. Therepeated differentiation of the trunk into two batches of segmentsfrom the homonomous trunk condition indicates an evolutionarytrend in aspects of body patterning regulation that was achievedindependently in several trilobite clades. The phylogeneticplacement of trilobites and congruence of broad patterns oftagmosis with those seen among extant arthropods suggest thatthe expression domains of trilobite cephalic Hox genes may haveoverlapped in a manner similar to that seen among extant arachnates.This, coupled with the fact that trilobites likely possessedten Hox genes, presents one alternative to a recent model inwhich Hox gene distribution in trilobites was equated to eightputative divisions of the trilobite body plan.     

The development of the nervous system in Laevicaudata (Crustacea, Branchiopoda): insights into the evolution and homologies of branchiopod limbs and ‘frontal organs’

We investigated the development of the external morphology and of the nervous system in Lynceus biformis and Lynceus brachyurus (Laevicaudata, Branchiopoda), by using immunohistochemical methods in combination with a confocal laser scanning analysis. In both Lynceus species, a free-swimming nauplius larva, equipped with three appendages, hatches from resting eggs. Despite their close phylogenetic relationship to each other, considerable differences are present in their external morphology. Hatching L. brachyurus larvae are equipped with a large and flattened labrum, where in contrast, the L. biformis larvae possess a smaller labrum with four conspicuous posteriorly directed spines at its margin. Despite these differences, the development of the nervous system is quite similar in both species. The hatching larvae are equipped with a naupliar nervous system, and only in the more advanced stages, the development of the ventral nerve cord starts. Furthermore, our investigation into the nervous system provided insights into architecture and evolution of protocerebral sensory organs, the dorsal setae field and the dorsal frontal organ, only present in Laevicaudata. The identification of frontal filaments with an associated frontal filament organ in Lynceus revealed—after a comprehensive comparison with other branchiopods—that these organs exist throughout Branchiopoda and are comparable to those in other crustaceans. Additionally, our results of the peripheral nervous system analysis showed that the innervation pattern of the naupliar appendages (antenna and mandible) and the trunk appendages could be serially homologized, despite much difference in gross morphology of these. Based on the innervation pattern of limbs, we suggest that the larval uniramous mandibular palp, found in the larvae of all ‘large’ branchiopods, is largely exopodal of nature (contrary to most earlier statements) and that the endopodite of the trunk limbs consists of only one distal endite-like segment (confirming some earlier statements) and not of three as proposed by others.     

On the Microanatomy of the Brain and the Innervation and Homologues of the Cephalic Appendages of Hesionidae and Syllidae (Polychaeta)

Abstract Earlier papers dealing with the anatomy of the hesionids and syllids were studied. Thereby it was found that information about the structure of the central nervous system was meagre. As a result, the anterior end appendages, especially the large, laterloventral ones of the Syllidae, have been differently interpreted. This prompted a re-investigation. The circum-oesophageal connectives, the brain commissures and the innervation of the alimentary canal and the cephalic appendages of a number of hesionid and syllid species were studied. The results, summarized in schematic diagrams, were compared with corresponding observations in other polychaete families. Among other things, it was concluded that not only the latero-ventral cephalic appendages of the hesionids but also those of the syllids are homologous with the palps of the nereids and of many other “errant” and “sedentary” families.     

A conserved genetic mechanism specifies deutocerebral appendage identity in insects and arachnids

The segmental architecture of the arthropod head is one of the most controversial topics in the evolutionary developmental biology of arthropods. The deutocerebral (second) segment of the head is putatively homologous across Arthropoda, as inferred from the segmental distribution of the tripartite brain and the absence of Hox gene expression of this anterior-most, appendage-bearing segment. While this homology statement implies a putative common mechanism for differentiation of deutocerebral appendages across arthropods, experimental data for deutocerebral appendage fate specification are limited to winged insects. Mandibulates (hexapods, crustaceans and myriapods) bear a characteristic pair of antennae on the deutocerebral segment, whereas chelicerates (e.g. spiders, scorpions, harvestmen) bear the eponymous chelicerae. In such hexapods as the fruit fly, Drosophila melanogaster, and the cricket, Gryllus bimaculatus, cephalic appendages are differentiated from the thoracic appendages (legs) by the activity of the appendage patterning gene homothorax (hth). Here we show that embryonic RNA interference against hth in the harvestman Phalangium opilio results in homeonotic chelicera-to-leg transformations, and also in some cases pedipalp-to-leg transformations. In more strongly affected embryos, adjacent appendages undergo fusion and/or truncation, and legs display proximal defects, suggesting conservation of additional functions of hth in patterning the antero-posterior and proximo-distal appendage axes. Expression signal of anterior Hox genes labial, proboscipedia and Deformed is diminished, but not absent, in hth RNAi embryos, consistent with results previously obtained with the insect G. bimaculatus. Our results substantiate a deep homology across arthropods of the mechanism whereby cephalic appendages are differentiated from locomotory appendages.