Ambulacra: plates with pore pairs through which the tube feet emerge.

Anal tube: prominent, plated, cone-like structure in crinoids that projects from the oral surface. The tube contains the anal opening.

Apical system: double ring of plates surrounding the periproct in echinoids.

Aristole's lantern: complex jaw-like mechanism usually found in regular, and some irregular, echinoids. Basal plates: lower ring of plates forming the crinoid calyx. Brachials: ossicles forming the arms in crinoids. Calyx: plated, cup-like structure in crinoids and blastoids containing the viscera. Also known as the theca. Columnals: ossicles forming the stem in crinoids. Dicyclic: crinoid calyces with an extra ring of plates, the infrabasals, between the basals and the stem. Fasciole: an area of the echinoid test that generates currents. Common in infaunal echinoids.

Genital plates: one of two types of plates forming the apical system. Plates have a hole through which gametes are released. Interambulacra: ossicles without perforations. Irregular echinoids: bilaterally symmetric echinoids, usually infaunal.

Madreporite: specialized genital plate through which water is drawn into the water vascular system.

Monocyclic: crinoid calyces with only one ring of plates between the stem and the radials, the basals.

Occular plate: one of two types of plates forming the apical system. Plates are part of the water vascular system. Ossicle: plate forming part of the echinoderm endoskeleton. Pedicellariae: tiny spines with pincers that remove settling organisms in echinoids.

Periproct: area surrounding the anus in echinoids. Peristome: area surrounding the mouth in echinoids. Pinnule: simple side branches of the crinoid arms. Plastron: flattened area behind the mouth in irregular echinoids with specialized paddle-shaped spines. Radial plates: upper ring of plates forming the calyx with an articulation site for the arms.

Regular echinoids: echinoids with pentaradiate symmetry. Stereom: microscopic lattice of rods permeated by tissue that forms the echinoderm plates.

Tegmen: cover for the oral surface in crinoids, sometimes soft, sometimes developed as a heavily plated "roof". Test: echinoderm endoskeleton.

Theca: plated, cup-like structure in crinoids and blastoids containing the viscera. Also known as the calyx. Tube feet: lateral extensions of the radial water vessels used in locomotion, respiration, and food gathering. Viscera: echinoderm main body organs.

Water vascular system: complex hydraulic system unique to echinoderms, principally used in locomotion and feeding.

Trilobites are arthropods, a phylum that includes insects and crustaceans.

Between the Cambrian and the Permian they were amongst the most important elements of marine communities. Their body plan, with flexible segments and many limbs, allowed them to occupy a wide variety of ecological niches. Trilobites went extinct in the end-Permian mass extinction event, but had been in decline throughout the Upper Palaeozoic.

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