An animal clade uniting groups with trochophore-like larvae (molluscs, annelids, ribbon worms, etc.) and lophophorate animals (bryozoans, brachiopods, etc.). Neurons expressing neuropeptides carrying GLWamide on the C terminus. An animal clade uniting moulting animals (nematodes, priapulids, arthopods, etc.). The name comes from the fact that their mouth forms separately from the blastopore.Įcdysozoa. An animal group consisting of Ambulacraria and Chordata. The second major clade of Deuterostomia, including cephalochordates (amphioxus), tunicates (ascidians, larvaceans, etc.) and vertebrates.ĭeuterostomia. Bilateria are subdivided into Xenacoelomorpha, Deuterostomia and Protostomia.Ĭhordata. The phylogenetic group of bilaterally symmetric animals, consisting of three germ layers. The mode of gastrulation in which the lateral lips of the blastopore fuse in a slit-like fashion leaving two openings: an anterior mouth and a posterior anus connected by a U-shaped gut.īilateria. In contrast to chordates, ambulacrarians do not have a centralized nervous system and, similar to non-deuterostome Bilateria, possess a ventral BMP signalling minimum.Īmphistomy. Ambulacraria includes echinoderms (sea urchins, starfish, etc.) and hemichordates. Besides chordates, one of the two major clades of Deuterostomia. A group of animals with a single gut opening previously thought to be members of flatworms, but now usually placed within the earliest branching bilaterian lineage Xenacoelomorpha.Īmbulacraria. These various types of symmetry and body axes raise the question of how and when bilaterality – a trait that likely contributed to the diversification of body plans (see Box 2) – might have arisen.Īcoela. Placozoans are irregularly shaped, crawling animals that exhibit a dorsal and ventral surface, although how these surfaces arise is unclear as placozoan embryogenesis is unknown. Comb jellies (Ctenophora) are bi-radially symmetric, with an oral-aboral axis and two other planes of symmetry, one going through the bases of the tentacles and the other through the slit-like mouth. For example, sponges (Porifera), although missing a clear body symmetry in their modular, sessile adult state, have an obvious radial symmetry as larvae. However, among the non-bilaterian Metazoa (see Glossary, Box 1), other types of symmetry exist ( Fig. 1). Bilaterality also favours the formation of left-right asymmetry in many animals, including vertebrates. The anterior end is usually characterized by a concentration of sensory organs, such as eyes and the olfactory system. For example, the central nervous system develops at the dorsal side of the vertebrate body, but ventrally in insects and many other animals. Body axes can be thought of as systems of molecular coordinates ( Niehrs, 2010), allowing different parts of the body to develop differently. an anterior-posterior axis and a dorsal-ventral (D-V) axis. Most animals belong to Bilateria (see Glossary, Box 1), a group encompassing organisms with three germ layers (ectoderm, endoderm and mesoderm) and two body axes, i.e. Here, we compare the molecular control of body axes in radially and bilaterally symmetric cnidarians and bilaterians, identify the minimal set of traits common for Bilateria, and evaluate whether bilaterality arose once or more than once during evolution. Recent findings from non-bilaterian animals, in particular from Cnidaria, the sister group to Bilateria, have shed new light into the evolutionary origin of bilaterality. Therefore, how bilaterality evolved and whether it evolved once or several times independently is a fundamental issue in evolutionary developmental biology. The emergence of bilaterality was a major evolutionary transition, as it allowed animals to evolve more complex body plans. These body axes are established during early embryogenesis and serve as a three-dimensional coordinate system that provides crucial spatial cues for developing cells, tissues, organs and appendages. Bilaterality – the possession of two orthogonal body axes – is the name-giving trait of all bilaterian animals.
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