A further distinctive echinoderm character, nearly certainly current in fossil taxa, is capture connective tissue, i beg your pardon is a kind of ligament the can change its nature from pliable come rigid, allowing parts the the skeleton, such as the stem or arms, to ‘lock’ into position for an extensive periods with minimal power expenditure.

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From: Encyclopedia of Geology, 2005

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A.B. Smith, in Encyclopedia that Geology, 2005

Phylogenetic Relationships

Echinoderms room triploblastic Metazoa, which, in spite of their unique pentaradiate adult human body plan, belong come the Bilateria. More specifically, echinoderms space members the the team Deuterostoma, i.e. Bilaterians in i beg your pardon the embryonic blastopore establishes into the anus quite than the mouth. This to adjust echinoderms with chordates and hemichordates, the latter consisting of acorn worms, pterobranchs, and also the extinct graptolites (see FOSSIL INVERTEBRATES | Graptolites). Recent molecular and morphological analyses unambiguously show that echinoderms and hemichordates space the more closely related, and also together these form the taxon Ambulararia. Both hemichordates and also echinoderms have a vascular device of pipe (the water vascular mechanism of echinoderms and the tentacular system of pterobranchs) that originates indigenous the homologous main body coeloms. In hemichordates the tentacular mechanism is symmetrically paired and also forms indigenous both the left and also the appropriate mesocoel. In echinoderms just the left mesocoel develops the water vascular system, and also hence over there is only a solitary hydropore. As both hemichordates and chordates room bilaterally symmetrical together adults and bear gill slits (pharyngeal openings), the latest typical ancestor that echinoderms and hemichordates must also have been bilaterally symmetrical and also have own gill slits.

Echinodermata are separated into five extant classes, which together kind the crown group of the phylum. These room the Crinoidea (feather stars and sea lilies) (see FOSSIL INVERTEBRATES | Crinoids), Asteroidea (starfishes), Ophiuroidea (brittle stars), Echinoidea (sea urchins) (see FOSSIL INVERTEBRATES | Echinoids) and Holothuroidea (sea cucumbers). Major differences different the body arrangement of crinoids indigenous those characterizing the other four classes, and also molecular and comparative data all allude to crinoids together the many primitive the the extant echinoderm classes. The crinoids and their extinction stalked loved ones the blastozoans together type the group Pelmatozoa. Asteroidea, Ophiuroidea, and also the Echinozoa kind a sister clade, the Eleutherozoa, and are free-living. Molecule evidence argues that amongst eleutherozoans the Echinoidea and Holothuroidea room the most closely related, and also together they kind the team Echinozoa. Echinozoa have actually a globular come cylindrical body form with the aboral surface extremely reduced. By contrast, asteroids and also ophiuroids have a stellate human body plan and their oral and aboral surfaces are equally developed. The die out edrioasteroids are an essential early eleutherozoan group.

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There are some numerically small but amazing stem-group echinoderms; most important among these room the carpoids (see below). Carpoids all absence pentaradiate symmetry, and also most lack an ambulacral mechanism while retaining pharyngeal gill slits.