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.     

Swimming and cleaning in the free-swimming phase of Argulus larvae (crustacea, branchiura)--appendage adaptation and functional morphology

The free-swimming early larval stages of Argulus foliaceus (Linneaus) (Branchiura) are studied using digital video, light microscopy, and SEM. We analyze and document the mode of swimming in the hatching stage of A. foliaceus and the subsequent juvenile stages with fully developed thoracopods. We present new observations and an analysis of the functional morphology of a cleaning behavior in the first stage. This stage swims very efficiently using the large exopods of the second antennae in concert with the mandibular palp (naupliar limbs), while the subsequent stages use the now developed thoracopods for propulsion. This posterior shift in propulsion is similar to--but independent from--what is seen in other crustaceans. The hatching stage has previously been referred as a "metanauplius" but as the first and second maxillae are developed and active, and buds of all four thoracopods are present, it is too advanced to be included in the naupliar phase. The hooks of the first antennae and the distal hooks of the maxillae are demonstrated to function not only as attachment organs (to the host), but also to play a significant role in the cleaning of the naupliar swimming appendages. A digital video-based analysis of the swimming mode is provided. The larval swimming pattern is generally similar to that of other crustaceans such as Branchiopoda and Cirripedia, but autapomorphies of the Branchiura include the following: 1) While actively swimming, the naupliar appendages are almost straight during the recovery stroke and 2) they have a relatively small deflection during movement ( approximately 25 degrees or approximately 35 degrees for mandible and second antenna respectively), 3) the larval mandible has a uniramous palp which is the retained exopod. The morphological implications of the transition from the possibly nonfeeding pelagic, or free-swimming, first larval stage to the feeding, parasitic second stage are discussed and compared with other crustaceans.     

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.     

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.     

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 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.     

Crustacea with preserved soft parts from the Upper Cambrian of Sweden

Six monotypic new genera of small crustaceans with phosphatised integument are described from Upper Cambrian, mainly bituminous limestone of southern Sweden. These are Dala peilertae, Bredocaris admirabilis, Walossekia quinquespinosa, Rehbachiella kinnekullensis, Skara anulata and Oelandocaris oelandica. The well-preserved details reveal a nauplius-like organisation of the head region. At least in Bredocaris, Walossekia , and Rehbachiella the head tagma has only three to four specialised pairs of appendages; Skara and Oelandocaris have five pairs of headal limbs. The antennae and the mandibulae served for locomotion and for mastication. All species have a filter apparatus with a distinct filter groove and serially constructed appendages. Four of the six forms had developed a conspicuous median compound eye. Specialisation in the development of the limbs, etc., suggests that some of the forms were benthie, while others may have been epibcnthic and active swimmers, Crustacea , Dala, Bredocaris, Walossekia, Rehbachiella, Skara, Oelandocaris, soft-part preservation, Upper Cambrian, Sweden.     

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.     

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 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 second type A-nauplius from the Upper Cambrian 'Orsten' of Sweden

Walossek, Dieter & Müller, Klaus J. 1989 07 15: A second type A-nauplius from the Upper Cambrian 'Orsten' of Sweden. Lethaia , Vol. 22, pp. 301–306. Oslo. ISSN 0024–1164.
Among newly sorted material, two specimens at first considered as nauplius-like larva A (Müller & Walossek 1986, Transactions of the Royal Society of Edinburgh: Earth Sciences 77 ) differ from this type in their larger sized body and appendages, much smaller caudal spines, and lack of the dorsal hook-like plates. They are regarded as a similar but distinct type, named 'larva A2'. * Crustacea, ontogeny, phosphatization, 3D-preservation .     

On the ontogeny of Leptodora kindtii (Crustacea,Branchiopoda, Cladocera), with notes on the phylogeny of the Cladocera

Leptodora kindtii, a large predaceous cladoceran, is among the most deviant species of the Cladocera. Therefore, its phylogenetic position has traditionally proven difficult to determine. Its many peculiar features include, among others, long, stenopodous, forwardly directed trunk limbs, a posteriorly placed dorsal brood pouch, a tri-lobed lower lip, and a long, segmented abdomen. This study describes the ontogeny of L. kindtii (Haplopoda), including general body proportions, appendages, the carapace, and other external structures in an attempt to facilitate the comparison of its aberrant morphology to that of other branchiopods. In general, the early embryos are similar to the early embryos of other cladoceran taxa with respect to body shape and size and position and orientation of the early limb buds. Many of the unusual features of L. kindtii appear late in ontogeny. The carapace appears at an early stage as a pair of dorsolateral swellings in a position corresponding to the gap between the mandibles and the first pair of trunk limbs; it later becomes posteriorly transposed by a gradual fusion of its more anterior parts to the dorsal side of the thorax. The tri-lobed "lower lip," under the labrum of the late embryo and the adult, develops as a fusion of the first maxillae (lateral lobes) to an elevated sternal region behind the mouth (median lobe). The stenopodous, segmented trunk limbs in the adult develop from embryonic, elongate, subdivided limb buds, similar to those seen in early stages of other branchiopods. Two conflicting possibilities for the phylogeny of the Cladocera, involving two different positions of L. kindtii (Haplopoda), are discussed. Several characters support a sister-group relationship between the Haplopoda and Onychopoda. However, some characters support the Anomopoda and Onychopoda as sister groups, leaving the Haplopoda outside this clade. In contrast to recent suggestions, we prefer to retain the term "Cladocera" in its original sense as comprising the Haplopoda, Ctenopoda, Anomopoda, and Onychopoda.     

Limb ontogeny and trunk segmentation in Nebalia species (Crustacea, Malacostraca, Leptostraca)

The ’egg-larval’ development of two species of Nebalia has been examined with SEM. Various details concerning limb ontogeny and trunk segmentation are described. The most important of these are the following. The tripartite state of the peduncle of antenna 2 in the adult of Nebalia species is derived from the fusion of the third and fourth podomeres, present in late larvae. The proximal portion of the mandible in the adult of Nebalia brucei, carrying the ’coxal process’, is, based on the ontogenetic evidence, interpreted as the combined basis and coxa, and the bipartite palp is interpreted as the endopod. The early development of the thoracopods and the three anteriormost pleopods is identical. They all start as laterally directed, biramous limb buds. This suggests that tagmatisation of the trunk of the Leptostraca (and other Malacostraca) has been developed from an ancestor with an undivided trunk region with serially similar limbs. Certain early stages reveal an extra, ’eighth’, limbless pleon segment, as compared with the normal number of seven pleomeres of adult Leptostraca. The presence of a row of ventral, sternitic, triangular processes between the bases of the thoracopods, as they are found in certain stages of a species of Nebalia, is suggested as a possible ground pattern for the Malacostraca. Accepted: 1 February 2000     

Martinssonia elongata gen. et sp.n., a crustacean-like euarthropod from the Upper Cambrian 'Orsten' of Sweden

A tiny arthropod, with five growth stages, is described. Three of the instars are metanauplius-like larvae, having unsegmented bodies and four pairs of appendages. The largest stage, with a length of about 1.5 mm, may still be immature. Its body is divided into three tagmata. The cephalon, including five appendiculate segments, h a projecting forehead with a rostral spine and a small shield with a joint between fourth and fifth segments. Eyes are absent. The trunk is composed of seven annular segments, the anterior two with appendages. The caudal end is a long pleotelson-like segment with the anus on its ventral surface. There are seven pairs of appendages: uniramous antennulae, composed of few tubular podomeres; four pairs of biramous postantennular, almost homeomorphic cephalic appendages; two pairs on the trunk, the anterior pair being similar to the cephalic appendages except for the exopodite, the posterior being much smaller, uniramous and apparently rudimentary. Martinssonia was probably benthic, feeding on detritic particles which it stirred up from the bottom. Besides various crustacean-like features, the new form reveals structures different from Crustacea as well as from all other known arthropodan groups. Martinssonia presumably is a descendant of an euarthropodan group, originating from the crustacean branch long before reaching the eucrustacean level of evolution.     

Captopodus poschmanni gen. et sp. nov. a new stem-group arthropod from the Lower Devonian Hunsrück Slate (Germany)

A new arthropod from the Lower Devonian Hunsrück Slate is described on the basis of four specimens. The body of Captopodus poschmanni comprises a head, a trunk with an anal portion. The high number of trunk appendages (≥66 segments) is unusual. The function of one pair of cupola-like structures of the head shield is unclear. The presence of large grasping appendages in the head superficially resembles the ‘short great appendages’ of other euarthropods and grasping appendages of thylacocephalans. The phylogenetic position of the arthropod cannot be determined in detail, though several morphological aspects indicate a phylogenetic position as a stem lineage representative of the Euarthropoda, the morphology of the trunk appendages seem to indicate a more advanced phylogenetic position. This new taxon underlines the exceptional diversity of arthropods within the Hunsrück Slate in comparison to other Devonian fossil sites and highlights the significance of the Hunsrück Slate for the evolution of early arthropods.     

Oviposition site selection by herbivorous beetles: a comparison of two thistle feeders, Cassida rubiginosa and Henosepilachna niponica

In insects that feed on plants in both adult and larval stages, it is often difficult to distinguish oviposition preference from adult feeding preference, because oviposition can occur at or in proximity to feeding sites. In the present study, characteristics of oviposition site selection of two beetle species, Cassida rubiginosa Müller (Coleoptera: Chrysomelidae) and Henosepilachna niponica (Lewis) (Coleoptera: Coccinellidae), were investigated in the field and laboratory, with particular attention to relationships with adult feeding sites. In the field, distances between adult feeding scars and egg masses differed for C . rubiginosa and H . niponica , with the former being very small and the latter averaging 24.6 cm. The same tendencies for the distances between adult feeding scars and egg masses of the two beetle species were confirmed in cages in which only female beetles were released. Cassida rubiginosa restricted egg laying to host plants in the field and to leaves in laboratory assays. On the other hand, H . niponica placed 8% of egg masses on plants adjacent to host plants in the field and often placed eggs on artificial substrates rather than leaf discs in laboratory assays. These results suggest that oviposition and female feeding sites are virtually inseparable in the case of C . rubiginosa , while H . niponica females do not necessarily keep to host plant leaves as oviposition substrates and they tend to oviposit at some distance from their feeding sites. Results are discussed in relation to proximate and ultimate causes of host selection behavior.     

A new species of the genus Lightiella: the first record of Cephalocarida (Crustacea) in Europe

A new species of Cephalocarida belonging to the genus Lightiella is described. Like all known species of Lightiella , the new species is characterized by reduction of trunk segment 8, which also lacks both pleura and thoracopods. The diagnostic characters of the species are: (1) one seta on the inner distal corner of the penultimate endopodal segment of second maxilla and thoracopods 1–5; (2) only one claw on the distal segment of the endopod of thoracopod 6. A cladistic analysis of 27 morphological characters was used to estimate the phylogeny of all species of Lightiella , with all other cephalocarid species used as outgroups. The discovery of this species in the Mediterranean fills a gap in the distribution of the genus and of the entire class.  © 2006 The Linnean Society of London, Zoological Journal of the Linnean Society , 2006, 148 , 209–220.     

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.