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.     

Reproductive strategy of an isopod Onisocryptus ovalis,parasitizing a bioluminescent myodocope ostracod Vargula hilgendorfii

The functional morphology and the reproductive strategy of a parasitic isopod Onisocryptus ovalis in a bioluminescent ostracod Vargula hilgendorfii as its final host were studied based on video and SEM observations. During its lifetime, Onisocryptus ovalis dramatically metamorphoses several times, changing sex from male to female in the final host's carapace. At nearly the last ontogenetical stage, the parasite anchors its body with a pair of thoracopods to the posterodorsal region of the host ostracod's trunk and loses all the other appendages and thus its mobility as well. Thereafter, the parasite reverses bodily orientation during the final moulting so as to locate its mouth in the midst of the host eggs, and finally consumes them, leaving only the egg membrane. Such a mode of feeding of the parasite following the fixation of the body is interpreted in terms of the adaptation to escape elimination from the ostracod carapace by the host's cleaning appendages (the seventh limbs) and to obtain as much space as possible for the parasite's own eggs/embryos at the sacrifice of the mother's mobility. The synchronization between the timing of metamorphosis of the parasite and the reproductive cycle of the host animal can be expected to guarantee the parasite the opportunity to exploit sufficient nutrition from the eggs of the host.     

The ontogeny of the cypridid ostracod Eucypris virens (Jurine, 1820) (Crustacea,Ostracoda)

The chaetotaxy (shape, structure and distribution of setae) of appendages and valve allometry during the post embryonic ontogeny of the cyprididine ostracod Eucypris virens are described. It is shown that the basic ontogenetic development of E. virens is very similar to that of other species of the family Cyprididae. During ontogeny, the chaetotaxy shows continual development on all podomeres of the limbs with the exception of the last podomere on the antennulae. The long setae on the exopodite and protopodite of the antennae have a natatory function until the actual natatory setae develop in later instars. Aesthetascs (presumed chemoreceptors) ya and y3 are the first to develop and may have an important function in the first instars. Cyprididae require a pediform limb in the posterior of the body presumably to help them to attach to substrates and this is reflected by the pediform nature of one limb at all times throughout all instars. This study has also shown that the fifth limb is most probably of thoracic origin and hence ostracods have only one pair of maxillae.     

SEM studies on the early larval development of Triops cancriformis (Bosc)(Crustacea: Branchiopoda, Notostraca)

We investigated early larval development in the notostracan Triops cancriformis (Bosc, 1801–1802) raised from dried cysts under laboratory conditions. We document the five earliest stages using scanning electron microscopy. The stage I larva is a typical nauplius, lecithotropic and without trunk limbs. The stage II larva is feeding and has trunk limb precursors and a larger carapace. Stage III larvae have larger trunk limbs and a more adult shape. Stage IV larvae have well developed trunk limbs, and stage V larvae show atrophy of the antennae. We describe the ontogeny of selected features such as trunk limbs and carapace, discuss ontogeny and homologization of head appendages, follow the development of the feeding mechanism, and discuss trunk limb ontogeny.     

The Ontogeny of Neonesidea oligodentata (Bairdioidea, Ostracoda, Crustacea)

This is the first detailed ontogenetic study of the appendages and carapace of a bairdioidean ostracod. This paper uses the development of the appendages and changes in the pore systems of the carapace through ontogeny to help determine the relationship between the Bairdioidea and other podocope groups. Neonesidea oligodentata has eight post-embryonic stages: one fewer than the Cypridoidea, Cytheroidea and Darwinuloidea. The first instar of N. oligodentata resembles that of the second instar of the Cypridoidea and Cytheroidea in terms of appendages, and it is postulated that there is an additional instar stage of N. oligodentata that molts within the egg. The general sequence of appearance of the limbs from instar A-7 onwards is similar to that of the Cypridoidea and Cytheroidea, but different from that of the Darwinuloidea. Like the Cypridoidea and Cytheroidea, N. oligodentata has a gap in its ontogenetic development during instar A-6, where no new Anlage is added. Pore system analysis of A-7 instars suggests that the Bairdioidea may be more closely related to the Cypridoidea than to the Cytheroidea.     

Limbs and tail as evolutionarily diverging duplicates of the main body axis

SUMMARY Contrasting hypotheses have been proposed to explain the pervasive parallels in the patterning of arthropod and vertebrate appendages. These hypotheses either call for a common ancestor already provided with patterned appendages or body outgrowths, or for the recruitment in limb patterning of single genes or genetic cassettes originally used for purposes other than axis patterning. I suggest instead that body appendages such as arthropod and vertebrate limbs and chordate tails are evolutionarily divergent duplicates (paramorphs) of the main body axis, that is, its duplicates, albeit devoid of endodermal component. Thus, vertebrate limbs and arthropod limbs are not historical homologs, but homoplastic features only transitively related to real historical homologs. Thus, the main body axis and the axis of the appendages have distinct but not independent evolutionary histories and may be involved in processes of homeotic co-option producing effects of morphological assimilation. For instance, chordate segmentation may have originated in the posterior appendage (tail) and subsequently extended to the trunk.     

Comparative morphology among trunk limbs of Caenestheriella gifuensis and Leptestheria kawachiensis (Crustacea: Branchiopoda: Spinicaudata)

Morphological differences among groups of the 24 trunk limbs of Caenestheriella gifuensis (Ishikawa, 1895) and differences between males and females are described and illustrated. A setose attenuate lobe located proximally near enditic lobe 1 and a discoid lobe covered with small setae proximal to enditic lobe 1 are newly described. The five ventral enditic lobes, endopod, exopod, and dorsal exite of traditional spinicaudatan morphology are redescribed. Trunk limbs 1–4 of females bear a palp on enditic lobe 5 and trunk limbs 1–15 of males bear a similar palp. A second, articulating palp is associated with the base of the endopod of trunk limbs 1–2 of males. The proximal part of trunk limbs 19–24, bearing enditic lobe 1, articulates by an arthrodial membrane with the remainder of the limb, and the exite is distal to this arthrodial membrane. Development of trunk limbs, ascertained through an examination of early juvenile instars of Leptestheria kawachiensis Uéno, 1927, includes an asetose limb followed in time by a series of setose limbs that increase in morphological complexity with age. The number of lobes on the asetose limb varies from seven (corresponding to five enditic lobes, an endopod, and an exopod) on anterior limbs to five on trunk limb 24, which lacks the lobes corresponding to enditic lobe 4 and the endopod; these two structures are added later to setose limbs. The attenuate lobe, the discoid lobe, the exite of all trunk limbs, and the palps of the anterior trunk limbs are added to the setose limbs. Development of anterior limbs is accelerated relative to that of posterior limbs, and development of the more posterior limbs is truncated relative to that of limbs immediately anterior to them. Enditic lobe 4 and the endopod of limbs like trunk limb 24 develop from, or are patterned by, enditic lobe 5; the articulating palp of male trunk limbs 1–2 also may be added in this way. A comparison of these observations with development of the copepod maxilliped suggests that the spinicaudatan trunk limb is composed of a praecoxa with three lobes, a coxa and a basis each with one lobe, and an endopod of three segments in females and four in males. This is similar to the homology scheme previously proposed by Hansen in 1925. A critique is given of attempts to homologize parts of arthropod limbs based on developmental gene expression patterns. Stenopodal to phyllopodal transformations of maxillipeds in copepods provide a model at least partly applicable to spinicaudatans, and a ‘multibranched’ interpretation of spinicaudatan (and by extension branchiopodan) limb morphology is rejected. There is nothing intrinsic to the structure of the adult trunk limbs suggesting that they are similar to the adult limbs of the ancestral branchiopod or the ancestral crustacean, but early developmental steps of more posterior limbs are good matches for the morphology of an ancestral crustacean biramal limb predicted by a hypothesis of duplication of the proximo‐distal axis. © 2003 The Linnean Society of London, Zoological Journal of the Linnean Society, 2003, 139 , 547–564. No claim to original US government works.     

The larval development of an Australian limnadiid clam shrimp (Crustacea, Branchiopoda, Spinicaudata), and a comparison with other Limnadiidae

The adult morphology of the Australian Limnadopsis shows some remarkable differences to that of other Limnadiidae. These differences are not reflected in its larval development. In Limnadopsis parvispinus, larval development comprises six stages. In stages I and II only the three naupliar appendages are present: the antennule as a small bud, the biramous antenna as the main swimming organ, and the mandible. The antennal protopod bears two endites, the proximal naupliar process and a more distal endite. In stage III a bifid naupliar process (in earlier stages not bifid) and the first signs of the carapace and trunk limb anlagen (undifferentiated rudiments) appear. In stage IV the carapace anlagen become more pronounced. The number of trunk limb anlagens increases to five, and differentiation has commenced. In stage V the first five pairs of trunk limbs are differentiated to varying degrees. The anterior-most four pairs of trunk limbs are subdivided into five endites, a small endopod, an exopod and an epipod. The bivalved carapace covers the anterior-most limbs. In larval stage VI the carapace is larger and the trunk limbs are further differentiated. A general pattern in the sequence of larval stages is the increasing number of sensilla on the antennules. From the last larval to the first postlarval stage, a significant change in morphology takes place. The trunk limbs are now used for swimming. Typical larval organs are much smaller than in the last larval stage. A comparison with other representatives of the Limnadiidae shows a high degree of correspondence, with most differences explained by the heterochronous appearance of characters during development. Five to seven stages are described for all studied Limnadiidae, including one particular stage in which four fully developed setae, a bifid naupliar process and the first signs of carapace anlagen are present. These characters are found in stage III in L. parvispinus, Limnadia stanleyana, Eulimnadia texana, and Imnadia yeyetta but in stage IV in E. braueriana and L. lenticularis. Based on a comparison of the larval stages of six limnadiid and one cyzicid species, we conclude that at least six naupliar stages belong to the limnadiid ground pattern.     

Larval development of Japanese 'conchostracans': part 1, larval development of Eulimnadia braueriana (Crustacea, Branchiopoda, Spinicaudata, Limnadiidae) compared to that of other limnadiids

As a part of a project to compare phylogenetically the larval or embryonic development of all major taxa of the Branchiopoda (Crustacea), the larval development of the Japanese spinicaudatan clam shrimp Eulimnadia braueriana Ishikawa, 1895, is described. Seven naupliar stages are recognized, based mainly on significant morphological differences between them, but in one case, on size alone. The seven stages range in length from 156 µm to 760 µm. Nauplius 1 is nonfeeding with incompletely developed and nonfunctional feeding structures. Nauplius 2 has apparently functional feeding structures, including a well-developed mandibular gnathobase, setulate protopodal endites of the antennae, and setules on various setae involved in swimming and food manipulation. Nauplius 3 is morphologically identical to Nauplius 2, but more than 50% larger. In nauplius 4, the coxal endite (naupliar process) of the antennae develops a bifid tip. Nauplius 5 has a lateral pair of primordial carapace lobes, and the first 4–5 pairs of trunk limb buds are weakly developed, making the anterior part of the trunk wider than the posterior. In nauplius 6, five pairs of trunk limb buds are visible externally and a small carapace has appeared, reaching approximately to trunk limbs 2; also, the pair of large buds behind the mandibles in previous stages has become divided into a large, anterior, setose bud and two smaller, posterior buds. The identities of these structures as either paragnaths or maxillules/maxillae remain uncertain. In nauplius 7, about six pairs of trunk limb buds are visible externally. The general morphology of the nauplius larvae of E. braueriana is much like those of the well-known Limnadia lenticularis (Linnaeus, 1758) and Eulimnadia texana Packard, 1871, including an elongate, lanceolate labrum; however, because of various heterochronies, the correspondence between the larval sequences of these species is not perfect. There is even less correspondence with the 5-stage larval development reported for Limnadia stanleyana King, 1855, and the spatulate labra of that species and Jmnadia spp. are different from those of other known limnadiid nauplii. The larvae of E. braueriana possess many typical (and synapomorphic) branchiopod features, such as the general morphology of the appendages involved in feeding and the mode of trunk limb development, while the small buds of the first antennae and the exact number and development of the parts of the trunk limbs are typical for the Spinicaudata.     

The Origin of the Crustacea*

Pre-Cambrian metamerically segmented bilaterians that ultimately gave rise to crustaceans probably arose from unsegmented flatworms. The recent suggestion that early arthropods, far from possessing a capacious segmented coelome of the annelid type, may never have had such, is attractive. Crustaceans were probably derived from small, segmented, surface-dwelling non-annelidan marine worms with a haemocoele. Their appendages probably originated as simple outgrowths whose shape was maintained by haemocoelic pressure. Possible routes whereby trunk limbs could have been derived from such rudiments are suggested. Trunk limbs would originally be unsegmented, as in many extant branchiopods and in certain Cambrian crustaceans. The evolution of thoracopodal feeding and some of the factors involved in the differentiation of the cephalic appendages are considered, as is the origin of the nauplius larva and the establishment of its feeding mechanism. Certain features of the cephalic region of the adult reflect changes necessitated as a result of the incorporation of the nauplius into the life cycle. Ontogeny would originally be anamorphic and follow the pattern preserved in its most primitive form in certain extant anostracan branchiopods. A reconstruction of the Ur-crustacean is attempted. Justification for features not previously associated with such a reconstruction, such as locomotory antennae, a relatively short trunk with only a short series of limbs and a limbless posterior region, and unsegmented trunk limbs, is provided by fossil evidence, functional considerations and the situation in primitive extant forms. Crustaceans were evidently not derived from any known arthropod clade. Stem lineage forms probably arose from the same group of pre-crustacean ancestors. While the Crustacea appears to be a monophyletic group, the idea that arthropodization must have occurred more than once and that the Arthropoda is a polyphyletic assemblage is supported, and evidence in favour of this view is cited.     

A eucrustacean from the Cambrian ‘Orsten’ of Sweden with epipods and a maxillary excretory opening

The Cambrian species Paulinecaris siveterae n. gen. n. sp., known from two trunk fragments, represents the first record of epipods (serving as gills and osmoregulatory structures) in a crustacean from the Swedish ‘Orsten’. Moreover, it is the first report of the maxillary excretory opening of a crustacean based on Cambrian material of ‘Orsten’‐type preservation. One specimen comprises the maxillary segment with an appendage and several thoracic segments with parts of their limbs; a second specimen is a fragment possibly of a more posterior part of the trunk. As in other known small eucrustaceans, the tergites of the new species lack prominent tergopleurae, so that the limbs insert directly ventral to the tergal margins. Limb preservation includes the maxilla and several thoracopods, all possessing a prominent, fleshy basipod with six setose endites along their median rim distally to the proximal endite. The presence of long and prominent limbs of P. siveterae suggests that it had good swimming ability, while the slight C‐like curvature of their basal limb part, basipod, indicates involvement of the limbs also in so‐called ‘sucking chambers’ for suspension feeding coupled with locomotion. The estimated total length of P. siveterae, 2–3 mm, is comparable to that of extant cephalocarids, but its appendages are twice as long and wide. The limbs of P. siveterae also differ in size and armature from extant eucrustaceans as well as early representatives of this group known from the ‘Orsten’ assemblages. The general morphology of the limbs, for example in having a fleshy and C‐shaped basipod with several setae‐bearing endites medially, identifies P. siveterae as an entomostracan eucrustacean, but a lack of further details precludes its affinity with any of the in‐group taxa. Three epipods on the outer edge of the basipod, as in P. siveterae, are also known from the Cambrian eucrustacean Yicaris dianensis from China and early ontogenetic stages of extant fairy shrimps (Anostraca); their adult stages have two epipods. This hints at an original number of three epipods in the ground pattern of Entomostraca, but some uncertainty remains with regard to the eucrustacean ground pattern because Malacostraca possess a maximum number of two.     

The feeding mechanisms of Lynceus (Crustacea: Branchiopoda: Laevicaudata), with special reference to L. simiaefacies Harding

Although generally assumed to be filter feeders, branchiopod crustaceans of the laevicaudatan genus Lynceus O.F. Müller, 1776 possess no filters and do not collect food by filtration. Investigated species of these bivalved, multi‐limbed animals have basically benthic habits and collect particulate food, mostly detritus, by scraping or sweeping it from surfaces with suitably armed trunk limbs. L. simiaefacies Harding, 1941, known only from a desert pool in Yemen, has trunk limbs that are armed with particularly robust scrapers and much of the complexity of these limbs and their armature is related to the collection and manipulation of detrital food by mechanical means. Material collected by scrapers borne distally on the more anterior limbs – although the anteriormost is very lightly armed – is swept posteriorly and dorsally, assisted by the armature of the more proximal endites, towards the posterior end of a deep food groove, whence it is passed anteriorly by the substantial gnathobases of the trunk limbs. The necessary movements of the trunk limbs are facilitated by a system of intrinsic muscles that enable individual endites to be moved independently – a remarkable specialized feature of a phyllopodial appendage. Before it enters the food groove, collected material is at all times confined to a narrow median chamber, or cage, between the two sets of opposed trunk limbs that extends over most of the anterior limbs – which are the largest. Each cage wall serves as a screen, covering the limbs of its side and is made up of long setose screening setae that superficially resemble coarse filter setae, and arise from the more proximal endites of most of the anterior trunk limbs. The screens prevent collected material from entering the inter‐limb spaces into which water flows during each cycle of trunk limb movements, where its presence would be disastrous. They do not interfere with the spines of the proximal endites that can protrude between them. The screens do not extend to the extreme posterior end of the trunk limb series where a complex and dense array of specialized spines of the short posterior trunk limbs completes the task of sweeping food material into the food groove. Material is passed anteriorly along the food groove by the trunk limb gnathobases and the small but robustly armed maxillules to the mandibles. Although constructed on the basic, boat‐like, branchiopod plan, in contrast to those of most particle‐feeding branchiopods whose mandibles have a broad masticatory surface, those of Lynceus have a masticatory surface that is narrow and elongate in the antero‐posterior plane. Interestingly, while the number of ‘teeth’ into which this surface is elaborated is few in most species of the genus, inviting comparison with a similar attribute in the Notostraca, L. simiaefacies has more numerous, smaller teeth. Although following the branchiopod plan, the mandibular musculature appears to have its own distinctive features but remains to be investigated in properly fixed material. At its distal extremity the oesophagus is differentiated into a small but complex gizzard, of which there appears to be no parallel in any other branchiopod order. This is described for the first time. Although provided with natatory antennae, species of Lynceus also employ their trunk limbs as organs of propulsion. In L. gracilicornis (Packard, 1871) the carapace valves can gape to more than 90°, which allows the trunk limbs to make a contribution to propulsion in a manner akin to that of the Anostraca. In this respect the Laevicaudata appears to stand in contrast to the Spinicaudata, in most species of which the trunk limbs contribute little or nothing to locomotion. More information is needed on representatives of both orders, which have received little study as living animals. Brief comments are made on the systematic position of the Laevicaudata, about which much remains to be resolved. © 2009 The Natural History Museum. Journal compilation © 2009 The Linnean Society of London, Zoological Journal of the Linnean Society, 2009, 155 , 513–541.     

Homology of Holocene ostracode biramous appendages with those of other crustaceans: the protopod, epipod, exopod and endopod

Unambiguously biramous appendages with a proximal precoxa, well-defined coxa and basis, setose plate-like epipod originating on the precoxa, and both an endopod and exopod attached to the terminal end of the basis are described from several living Ostracoda of the order Halo-cyprida (Myodocopa). These limbs are proposed as the best choice for comparison of ostracode limbs with those of other crustaceans and fossil arthropods with preserved limbs, such as the Cambrian superficially ostracode-like Kunmingella and Hesslandona. The 2nd maxilla of Metapolycope (Cladocopina) and 1st trunk limb of Spelaeoecia, Deeveya and Thaumatoconcha (all Halocypridina) are illustrated, and clear homologies are shown between the parts of these limbs and those of some general crustacean models as well as some of the remarkable crustacean s.s. Orsten fossils. No living ostracodes exhibit only primitive morphology; all have at least some (usually many) derived characters. Few have the probably primitive attribute of trunk segmentation (two genera of halocyprid Myodocopa, one order plus one genus of Podocopa, and the problematic Manawa); unambiguously biramous limbs are limited to a few halo-cyprids. Homologies between podocopid limbs and those of the illustrated primitive myodocopid limbs are tentatively suggested. A setose plate-like extension, often attached basally to a podocopid protopod, is probably homologous to the myodocopid epipod, which was present at least as early as the Triassic. Somewhat more distal, less setose, and plate-like extensions, present on some podocopid limbs (e.g., mandible), may be homologous instead to the exopod (clearly present on myodocopid mandibles). The coxa (or precoxa) is by definition the most basal part of the limb. A molar-like tooth is present proximally on the mandibular protopod of many ostracodes; it is the coxal endite and projects medially from the coxa (or proximal protopod). The Ostracoda is probably a monophyletic crustacean group composed of Myodocopa and Podocopa. All have a unique juvenile (not a larva) initially with three or more limbs. Except that juveniles lack some setae and limbs, they are morphologially similar to the adult. Thus the following suite of characters in all instars may be considered a synapomorphy uniting all Ostracoda: (1) Each pair of limbs is uniquely different from the others. (2) The whole body is completely enclosed within a bivalved carapace that lacks growth lines. (3) No more than nine pairs of limbs are present in any instar. (4) The body shows little or no segmentation, with no more than ten dorsally defined trunk segments. No other crustaceans have this suite of characters. A probable synapomorphy uniting the Podocopa is a 2nd antenna with exopod reduced relative to the endopod.     

The ontogeny of appendages and carapace of Neonesidea schulzi (Ostracoda: Bairdiidae) from the Red Sea coast,Egypt

The ostracod genus Neonesidea is broadly distributed in shallow marine waters. The ontogeny of the N. schulzi (Bairdiidae) is described in detail by studying the development of the appendages and variations in carapace form, size and structure. Neonesidea schulzi has eight post-embryonic instars, and a gap in its ontogenetic development during instar A-6, where no new Anlage is added. The Anlagen of the copulatory organs and the forked terminal claw of second antenna appear in the seventh (A-1) instar, and the first thoracic legs of podocopid ostracods are shown to descend from the thoracic region. For the first time in ostracods, observations of moulting from sixth and seventh instars are presented.     

The ontogeny of appendages of Heterocypris salina (Brady, 1868) Ostracoda (Crustacea)

The post-embryonic development of the appendages of the Cyprididae ostracod Heterocypris salina (Brady, 1868) are described in detail and compared with those of other podocope species documented in previous studies. Generally, the appearence of limbs during onotgeny of H. salina is similar to that of other species, but small differences in limb morphologies were identified between H. salina and other Cyprididae species, including other Heterocypris species. Some features appear either earlier or later in the development of H. salina compared with other species, even species of the same genus. These features may be useful characters for phylogenetic analyses at the genus and family levels.     

Brain anatomy of the marine tardigrade actinarctus doryphorus (arthrotardigrada)

Knowledge of tardigrade brain structure is important for resolving the phylogenetic relationships of Tardigrada. Here, we present new insight into the morphology of the brain in a marine arthrotardigrade, Actinarctus doryphorus, based on transmission electron microscopy, supported by scanning electron microscopy, conventional light microscopy as well as confocal laser scanning microscopy. Arthrotardigrades contain a large number of plesiomorphic characters and likely represent ancestral tardigrades. They often have segmented body outlines and each trunk segment, with its paired set of legs, may have up to five sensory appendages. Noticeably, the head carries numerous cephalic appendages that are structurally equivalent to the sensory appendages of the trunk segments. Our data reveal that the brain of A. doryphorus is partitioned into three paired lobes, and that these lobes exhibit a more pronounced separation as compared to that of eutardigrades. The first brain lobe in A. doryphorus is located anteriodorsally, with the second lobe just below it in an anterioventral position. Both of these two paired lobes are located anterior to the buccal tube. The third pair of brain lobes are situated posterioventrally to the first two lobes, and flank the buccal tube. In addition, A. doryphorus possesses a subpharyngeal ganglion, which is connected with the first of the four ventral trunk ganglia. The first and second brain lobes in A. doryphorus innervate the clavae and cirri of the head. The innervations of these structures indicate a homology between, respectively, the clavae and cirri of A. doryphorus and the temporalia and papilla cephalica of eutardigrades. The third brain lobes innervate the buccal lamella and the stylets as described for eutardigrades. Collectively, these findings suggest that the head region of extant tardigrades is the result of cephalization of multiple segments. Our results on the brain anatomy of Actinarctus doryphorus support the monophyly of Panarthropoda. J. Morphol. 275:173–190, 2014. © 2013 Wiley Periodicals, Inc.     

Morpho-functional grounds of mode of life of Cladocera. V. Morphology and adaptive modifications of trunk limbs of anomopoda

Summary The structure of trunk limbs of Cladocera is comparatively reviewed. The structure of thoracic limbs of the primitive Chydorids Saycia cooki and Eurycercus glacialis is described and figured. The structure of leg I is described and figured in detail. The component parts of leg I of Chydoridae are homologized with those of other Anomopoda. The main phylogenetic trends are pointed out for leg I formation: involution of the posterior lobe of the endite, oligomerization of the setae in the homologous groups, especially those of the posterior lobes of the endite, specialization of the external branch of the endite for creeping in littoral forms and its general involution in planktonic forms. In connection with creeping the setae of the external branch of the endite differentiate in length and form, some of setules may enlarge or disappear. Legs II–V are built according to the same pattern but considerably differ in form and function in Cladocera, which are related to Conchostraca, possessing little differing appendages of different segments. The structure and the chaetal system of the homologous parts of legs II–VI of Anomopoda are analysed. Phylogenetic trends towards specialization and oligomerization are pointed out. The setae tend to form functional groups (e.g. in legs I, III). Thoracic legs II–VI present an example of a plasticity of a metameric organ built by the same pattern. These legs lost a locomotory function and perform other tasks. Setae of Cladocera have rather high formative possibilities. The examples of extreme changes of form are pointed out.     

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.     

A large new leanchoiliid from the Burgess Shale and the influence of inapplicable states on stem arthropod phylogeny

Characterized by atypical frontalmost appendages, leanchoiliids are early arthropods whose phylogenetic placement has been much debated. Morphological interpretations have differed, some of which concern critical characters such as the number of eyes and head appendages, but methodological approaches also have diverged. Here, we describe a new leanchoiliid, Yawunik kootenayi gen. et sp. nov., based on 42 specimens from the newly discovered Marble Canyon locality of the Burgess Shale (Kootenay National Park, British Columbia; middle Cambrian). This new morphotype demonstrates the presence of a four‐segmented head in leanchoiliids, along with two small antero‐median eyes in addition to lateral eyes. Yawunik is characterized by a 12‐segmented trunk and a carinate, lanceolate telson adorned with minute spines. The ‘great appendages’ of the animal bear teeth on their two distal rami, which would have enhanced their ability to grasp prey. Attitudes of specimens, resulting from burial at multiple aspects of bedding, suggest the ‘great appendages’ were flexible and capable of antero‐posterior rotation. We also discuss the nature of intersegmental tissues and filaments present within the ‘great appendages’. Our phylogenetic analyses extend the monophyly of leanchoiliids to include Haikoucaris and Yohoia in a new clade, the Cheiromorpha nom. nov. (within Heptopodomera nom. nov.). Other nodes are poorly resolved unless implied weights are used, and in this case, the topology is critically sensitive to the coding prerogative of inapplicable states (NAs). Both the traditional ‘Arachnomorpha’ hypothesis (NAs as additional states) and the more recently favoured ‘Artiopoda + Crustacea’ (NAs as uncertainties) were obtained using the same data set and outgroup. This result stresses, first, the historical importance of polarization over data content in scenarios of early arthropod evolution, and second, a pressing need to investigate the impacts of coding inapplicables, especially given the inflating effect of implied weights.