Echinodermata Echinodermata

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Asteroidea Nervous system • the peripheral nervous system includes two intraepithelial nets, the sensory ectoneural system in the epidermis and the motor hyponeural system in the coelomic lining • the CNS consists of ectoneural and hyponeural circumoral nerve rings and radial nerves • The motor component of the radial nerve innervates the ampullae, tube feet, and body-wall muscles, while the sensory part receives input from the sense cells and organs • no ganglia, no glea Asteroidea Hemal system • three interjoined radial sets of vessels, each consisting of a hemal ring around the gut and five sometimes branched radial vessels • the hyponeural hemal ring, the gastric hemal ring, and the genital hemal ring • Two specialized vessels called gastric hemal tufts cross the perivisceral coelom and unite the gastric hemal ring with the axial hemal vessel and heart • the three hemal rings are united by the vertical axial hemal vessel (axial gland) that lies in the wall of the axial canal Ectoneural nerve net Hyponeural nerve net
3 Bilateral symmetry Anteroposterior body organization Medial nerve cord Gut and CNS parallel Complex life cycle with larva Ciliated band in larva b 1 Brachiolar complex (larva) 2 Rudiment ectoderm invaginates 3 Cilated band rearranged in larva 4 Radial symmetry Loss of cephalization and brain? Radial nerve cords Gut and CNS orthogonal Water vascular system with podia Calcareous endoskeleton Metazoa Regeneration and clonal reproduction Larvae Adults feather stars sea stars brittle stars sea urchins sea cucumbers Morphology (Crinoidea) (Asteroidea) (Ophiuroidea) (Echinoidea) Genes : sites of expression Asterozoa Echinodermat Eleutherozoa ry history of body architecture and regulatory genes within the hinodermata (b). Reconstructed changes in morphology (left) mental roles and expression domains of regulatory genes he expression domains and roles of several regulatory genes ropods and chordates (a, 8) 8–11 , despite their phylogenetic orphological disparity 2,5,6 . These similarities probably reflect architectural features, such as bilateral symmetry and an is (a, 3) between protostomes and deuterostomes. Morpholo- istinguish phyla (such as a,1) and groups of related phyla( a, 2) ome such cases, changes in the developmental roles of , 5 and 7) have been identified that may have been involved e derived structures 9,10,17 . The origin of echinoderms followed rthropods and chordates, and involved substantive modifica- ? Echinozo Sexual reproduction and development 7 Hox cluster domains along A/P axis distal-less anterior CNS engrailed ganglia along CNS orthodenticle 8 Hox genes domains along A/P axis 5 orthodenticle ciliated band (larva) 6 engrailed rudiment invagination 7 distal-less brachiolar arms (larva) distal-les subtrochal cells (larva 8 engrailed ectoderm (skeletogenesis) orthodenticle podia distal-les podia tions in basic body architecture (b, 2) 1–6 ; already present was a complex life cycle involving a larva that fed using a ciliated band (a, 3) 1–6 . Data reported here and taken from the literature 8–11,19 allow us to reconstruct changes in the expression domains of three regulatory genes during the origin and radiation of echinoderms. Several derived features of gene expression are shared by asterozoan and echinozoan echinoderm species (b, 7), implying that they were present before the divergence of these clades over 490 Myr ago 22 . The origin of these features could be even earlier (arrow near base of cladogram), but expression data are currently unavailable for crinoids, the most basal clade of extant echinoderms. Other features of gene expression appear to be limited to a single class (such as b, 4 and 6), suggesting later origins or subsequent losses within other lineages. Phylogenetic relationships are from refs 2, 6, 7 and 14. Egg Embryo Early larva Late larva Juvenile bilaterally symmetrical of echinoderm development. Echinoderm embryos and larvae etrical 1,6,16 . In late larvae, discrete populations of ectodermal cells on the left side of the body proliferate to form the ent (green crescent). The first radially symmetrical structures udiment (disc near late larva). During metamorphosis, many left-right asymmetrical radially symmetrical Echinoderm embryos and larvae are bilaterally symmetrical. In late larvae, discrete populations of cells on the left side of the body proliferate to form the imaginal adult rudiment. During metamorphosis, the rudiment gives rise to much of the radially metamorphosis. symmetrical juvenile (green). Lowe and Wray, 1997 Nature larval tissues (blue) are lost, and the rudiment gives rise to much of the juvenile (green). Domains of regulatory gene expression fall into three distinct symmetry classes: bilateral in embryos and early larvae; left–right asymmetric in late larvae, during establishment of the imaginal rudiment; and radial during and after Nature © Macmillan Publishers Ltd 1997 6 OCTOBER 1997 719 Sexual reproduction and development • Asteroids are typically gonochorists with external fertilization and a complex life cycle that includes a planktotrophic larva • Up to 2,500,000 eggs • Development includes radial cleavage, regulative development, and enterocoelous origin of the coelomic cavities • The bilaterally symmetric, suspensionfeeding larva (bipinnaria -> brachiolaria) Class Crinoidea crown stalk • The main part of the body is supported above the substrate either by a long stalk or by a series of grasping claws (cirri) • suspension feeders with mouth oriented upward • feather starts and sea lilies (~700 species) cirri
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Page 7: NATURE | VOL 392 | 5 MARCH 1998 b C

Echinodermata Echinodermata

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