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The origins and evolution of the Ecdysozoa

Maximilian J. Telford, Sarah J. Bourlat, Andrew Economou, Daniel Papillon, and Omar Rota-Stabelli

in Animal Evolution: Genomes, Fossils, and Trees

Ecdysozoa is a clade composed of eight phyla, three of which — arthropods, tardigrades, and onychophorans — share segmentation and have appendages, and the remaining five — nematodes, nematomorphs, ... More

Ecdysozoa is a clade composed of eight phyla, three of which — arthropods, tardigrades, and onychophorans — share segmentation and have appendages, and the remaining five — nematodes, nematomorphs, priapulids, kinorhynchs, and loriciferans — are worms with an anterior proboscis or introvert. Ecdysozoa contains the vast majority of animal species and there is a great diversity of body plans among both living and fossil members. The monophyly of the clade has been called into question by some workers based on analyses of whole genome datasets and we review the evidence that now conclusively supports the unique origin of these phyla. Relationships within Ecdysozoa are also controversial and we discuss the molecular and morphological evidence for several monophyletic groups within this superphylum.Less

A gallery of the major bilaterian clades

Alessandro Minelli

in Perspectives in Animal Phylogeny and Evolution

Current perspectives on the phylogeny of the major lineages of bilaterian animals are reviewed. Molecular phylogenetics suggests that the Acoela represent the most basally branching group of living ... More

Current perspectives on the phylogeny of the major lineages of bilaterian animals are reviewed. Molecular phylogenetics suggests that the Acoela represent the most basally branching group of living Bilateria. The still poorly circumscribed Annelida have been found to include the Pogonophora, the Echiura, and, tentatively, the Sipuncula. The Acanthocephala have emerged as modified Rotifera, with which they are now grouped as the Syndermata. The Phoronidea have been shown to derive from within the Brachiopoda, thus requiring be grouped with the latter in the monophyletic taxon Phoronozoa. The hexapods form a clade (Tetraconata) together with the crustaceans, to the exclusion of the myriapods, which possibly branch together with the chelicerates (Myriochelata). Still very problematic, but firmly within the protostomes, is the position of the Chaetognatha, whereas the Xenoturbellida, previously regarded as flatworms or molluscs, are now recognized as members of the deuterostomes.Less

Neurobiology of Perception of Illusory Contours in Animals

Andreas Nieder

in How Animals See the World: Comparative Behavior, Biology, and Evolution of Vision

This chapter describes the putative neurobiological mechanisms of illusory contour perception in vertebrates and invertebrates. Different animal species with divergent and independently evolved ... More

This chapter describes the putative neurobiological mechanisms of illusory contour perception in vertebrates and invertebrates. Different animal species with divergent and independently evolved visual systems perceive illusory contours. Animals of very different classes—such as owls and cats (Aves and Mammalia), and even very diverse phyla, like bees and monkeys (Arthropoda and Chordata)—perceive illusory contours. This is surprising, since the visual systems and pathways of these animals have evolved completely independently (in the case of insects and vertebrates) or at least largely independently (for instance, in birds and mammals) from each other. Clearly, there is a high selection pressure for animals of different taxa to see boundaries in the absence of contrast borders, and this constitutes an evolutionary advantage. Therefore, different species may have adopted convergent neural strategies to enable the perception of illusory contours.Less

PANARTHROPODA

Claus Nielsen

in Animal Evolution: Interrelationships of the Living Phyla

The Panarthropoda is a group of segmented ecdysozoans arranged into three phyla: Arthropoda, Onychophora, and Tardigrada. The panarthropods are almost unanimously regarded as monophyletic, and ... More

The Panarthropoda is a group of segmented ecdysozoans arranged into three phyla: Arthropoda, Onychophora, and Tardigrada. The panarthropods are almost unanimously regarded as monophyletic, and possess paired appendages and a cuticle containing a-chitin and protein, but no collagen. There are two main groups of panarthropod fossils: ‘lobopods’ and stem-group arthropods. Little is known about the morphology and life cycle of the panarthropod ancestor, with the embryology of tardigrades and onychophorans offering no clue. Almost all recent analyses, based on mitochondrial genomes, expressed sequence tag data, and phylogenomics indicate that onychophorans and arthropods are sister groups, although the position of the tardigrades remains uncertain.Less

Parasite-Crocodilian Database

Marisa Tellez

in A Checklist of Host-Parasite Interactions of the Order Crocodylia

This chapter lists all of the major parasite groups identified from crocodilians and lists the crocodilian species infected with the particular parasite species.

Phylum Arthropoda

Claus Nielsen

in Animal Evolution: Interrelationships of the Living Phyla

The Arthropoda is one of the largest phyla in the animal kingdom, with the insects alone purportedly consisting of several million living species. The other arthropods are estimated to be more than ... More

The Arthropoda is one of the largest phyla in the animal kingdom, with the insects alone purportedly consisting of several million living species. The other arthropods are estimated to be more than 100,000. The fossil record both of living and extinct groups dates back to the Early Cambrian and provides important insights about the evolution of the phylum. There are two or three subphyla comprised of a number of subgroups: Pycnogonida, Chelicerata (Xiphosura + Arachnida), and Mandibulata (Crustacea + Tracheata (= Hexapoda + Myriapoda)). However, more recent evidence from morphological and molecular studies point to Hexapoda as an ingroup of the Crustacea. The name Pancrustacea may be applied for the clade, while ‘crustaceans’ may be used for the non-hexapod groups. The Myriapoda appears to be the sister group of the Pancrustacea. The Pycnogonida has been interpreted as the sister group of the Euchelicerata (Xiphosura + Arachnida), while the Pentastomida has been placed within the Arthropoda as the sister group of the Branchiura.Less

Phylum Onychophora

Claus Nielsen

in Animal Evolution: Interrelationships of the Living Phyla

The phylum Onychophora consists of about 200 species of terrestrial animals that are mainly found in humid tropical and southern temperate regions, and comprises two families: Peripatidae and ... More

The phylum Onychophora consists of about 200 species of terrestrial animals that are mainly found in humid tropical and southern temperate regions, and comprises two families: Peripatidae and Peripatopsidae. The marine stem group of the living onychophorans appears to be represented by Lower and Middle Cambrian lobopodians, such as Ostenotubulus. Two species, Peripatopsis capensis and Peripatopsis balfouri, have well-characterised development. Several apomorphies indicate that the onychophorans are monophyletic. Almost all newer molecular phylogenies support the phylogenetic position of the Onychophora as the sister group of the Arthropoda.Less

Insects and Near Relatives

Patricia J. Vittum

in Turfgrass Insects of the United States and Canada: Third Edition

This chapter presents a taxonomy of insects. Insects and mites belong to a larger category of related animals, the phylum Arthropoda. Of all arthropods, mites and spiders are the most closely related ... More

This chapter presents a taxonomy of insects. Insects and mites belong to a larger category of related animals, the phylum Arthropoda. Of all arthropods, mites and spiders are the most closely related to insects in form and function and in their status as pests that damage turfgrass. The chapter then considers the form and function of insects and mites. It looks at the types of mouthparts and turf-feeding damage of chewing and sucking insects. The chapter also studies the orders of turfgrass-damaging insects and mites. Relatively few orders of insects and mites have species destructive to turfgrass. The orders Lepidoptera and Coleoptera are by far the most important in number of destructive species present.Less

PERSPECTIVE—Heads And Brains In Arthropods: 40 Years After The ‘Endless Dispute’

Gerhard Scholtz

in Structure and Evolution of Invertebrate Nervous Systems

The problem of arthropod head segmentation is addressed. In spite of a certain consensus concerning some features, such as a deutocerebral position of chelicerae, the head problem is still unsolved. ... More

The problem of arthropod head segmentation is addressed. In spite of a certain consensus concerning some features, such as a deutocerebral position of chelicerae, the head problem is still unsolved. In this chapter a new perspective on the issue is presented through the deconstruction of the head and brains of recent and fossil panarthropods. Three units are identified that reveal an independent evolutionary pace of cephalization: (1) dorsal cephalized structures such as head shields, carapaces etc., (2) ventral cephalized structures, such as appendages transformed to mouth parts or sensory organs, and (3) cerebralized regions of the central nervous system. Based on current phylogenetic hypotheses of panarthropod relationships, and new data concerning fossil brains, the evolutionary alterations of the three units are discussed. In particular, the phylogenetic positions of Tardigrada and of some fossil taxa, as well as the interpretation of fossil neuroanatomy, are crucial for the reconstruction of the evolution of arthropod heads.Less

Arthropoda

Heinz A. Lowenstam and Stephen Weiner

in On Biomineralization

The arthropods are distinguished by having segmented bodies and appendages, as well as hardened external skeletons. Growth is achieved by shedding the exoskeleton and then regenerating a new and ... More

The arthropods are distinguished by having segmented bodies and appendages, as well as hardened external skeletons. Growth is achieved by shedding the exoskeleton and then regenerating a new and larger one. The hardening of the exoskeleton usually occurs by chemical cross-linking (sclerotization) of the macromolecular constituents, mostly proteins and the polysaccharide, α-chitin. The major exception is the class Crustacea. The members of this group harden their skeleton not only by sclerotization, but also by the addition of inorganic minerals. After each molting, the new exoskeleton is remineralized. The result is that many Crustacea, particularly those that live in freshwater or on land where the availability of calcium is limited, have evolved novel and diverse temporary storage sites for mineral (reviewed by Greenaway 1985). From the perspective of biomineralization processes, this adaptation is certainly one of the “highlights” of the Arthropod phylum. Interestingly one taxonomic order within the Crustacea, the Cirripedia or barnacles, does not moult their heavily mineralized cuticles, even though their “organic” exoskeleton does go through periodic molting cycles (Darwin 1854). Table 7.1 lists many of the known reports of biomineralization processes in the Arthropoda. The table is already impressively long. However, as this phylum is by far the largest in the animal kingdom, we have no doubt whatsoever that the true extent of mineralization processes in the Arthropoda is far from having been ascertained. In the insects alone nearly half a million species have been described, and our list comprises just a few documented cases of insects that mineralize. Interestingly, the list of minerals formed by insects includes a number of so-called “organic minerals,” for example, uric acid, crystalline wax, and long chain paraffins. We strongly suspect that many more “organic minerals” have yet to be discovered among the insects. Less