![]() ![]() Deuterostome development and evolution are important for understanding the origin of chordates, but the scanty fossil record of enteropneust hemichordates and tunicates makes it difficult to visualize the ancestor ( Swalla & Smith, 2008).įig. Likewise, including new taxa and further phylogenomic analyses has suggested that the Acoela, Nemertodermatida, and Xenoturbella, all bilaterally symmetrical marine worms that lack several features common to most other bilaterians (for example, an anus, nephridia, and a circulatory system) are an outgroup to the bilateria ( Cannon et al., 2016). However, chaetognaths are now seen as an outgroup to the Lophotrochozoa (within the gnathiferan spiralian protostomes, Marlétaz, Peijnenburg, Goto, Satoh, & Rokhsar, 2019), and their deuterostome-like development has been attributed to a more ancestral-like pattern of development ( Martín-Durán, Passamaneck, Martindale, & Hejnol, 2016). Various animal groups have at one time or another been included in the deuterostomes due to developmental traits or phylogenomics, including chaetognaths, acoels and xenoturbella ( Bourlat et al., 2006 Swalla & Smith, 2008). The Deuterostomia cluster into two great clades-Chordata and Ambulacraria ( Fig. 12.1E), commonly known as sea cucumbers, with a body plan significantly extended in the oral–aboral axis and lacking arms. 12.1D), nearly exclusively comprising sessile animals, such as sea lilies and feather stars (sea lilies have a flower-shaped body, while feather stars have long arms with many branches) and the Holothuroidea ( Fig. 12.1C), commonly known as flat sand dollars and spherical sea urchins, lacking arms but possessing a typical pentamerous plan of echinoderms in five ambulacral areas the Crinoidea ( Fig. 12.1B), or brittle stars, with slender arms sharply set off from the central disc without lobes of the alimentary tract the Echinoidea ( Fig. 12.1A), commonly known as starfish or sea stars, typically possessing five arms, each of which contains gonads and part of the digestive tract the Ophiuroidea ( Fig. Currently, echinoderms are divided into five major classes: the Asteroidea ( Fig. The phylum of Echinodermata, comprising approximately 6000 extant species, 3 is an ancient group formed of benthic marine animals that live in both intertidal and deep sea benthos. It has been suggested that the echinoderms diverged from a common vertebrate ancestor before the beginning of the Cambrian period, over 600 million years ago. ![]() ![]() Due to the abundance and calcareous shells of echinoderms, these organisms have been well preserved as fossils. The echinoderms and the chordates are deuterostomes, while all other invertebrates are protostomes. In the deuterostomes, the blastopore can differentiate into the anus, while the mouth can develop from another embryonic area. In the protostomes, the blastopore differentiates into the mouth. 1,2 During the early Precambrian period, these animals split into two main groups: protostomes and deuterostomes. The deuterostomes, of which echinoderms, hemichordates, tunicates, and all higher chordates are the major extant groups, constitute a separate branch of the animal kingdom. ![]()
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