Editing Echinoderm (section)

===Skin and skeleton===

Echinoderms have a [[mesoderm]]al skeleton in the dermis, composed of [[calcite]]-based plates known as [[Ossicle (echinoderm)|ossicles]]. If solid, these would form a heavy skeleton, so they have a sponge-like porous structure known as stereom.<ref>{{harvnb|Behrens|Bäuerlein|2007|p=393}}</ref><ref>{{harvnb|Brusca|Moore|Shuster|2016|pp=979–980}}</ref> Ossicles may be fused together, as in the [[Test (biology)|test]] of sea urchins, or may [[Articulation (anatomy)|articulate]] to form flexible joints as in the arms of sea stars, brittle stars and crinoids. The ossicles may bear external projections in the form of spines, granules or warts and they are supported by a tough [[Epidermis (zoology)|epidermis]]. Skeletal elements are sometimes deployed in specialized ways, such as the chewing organ called "[[Aristotle's lantern]]" in sea urchins, the supportive stalks of crinoids, and the structural "lime ring" of sea cucumbers.<ref name=Ruppert2004p873 />

Although individual ossicles are robust and fossilize readily, complete skeletons of starfish, brittle stars and crinoids are rare in the fossil record. On the other hand, sea urchins are often well preserved in chalk beds or limestone. During fossilization, the cavities in the stereom are filled in with calcite that is continuous with the surrounding rock. On fracturing such rock, [[paleontology|paleontologists]] can observe distinctive cleavage patterns and sometimes even the intricate internal and external structure of the test.<ref>{{harvnb|Davies|1925|pp=240–241}}</ref>

The epidermis contains pigment cells that provide the often vivid colours of echinoderms, which include deep red, stripes of black and white, and intense purple.<ref name="Perillo Oulhen 2020">{{harvnb|Perillo|Oulhen|Foster|Spurrell|2020}}</ref> These cells may be light-sensitive, causing many echinoderms to change appearance completely as night falls. The reaction can happen quickly: the sea urchin ''[[Centrostephanus longispinus]]'' changes colour in just fifty minutes when exposed to light.<ref>{{harvnb|Weber|Dambach|1974}}</ref>

One characteristic of most echinoderms is a special kind of tissue known as [[catch connective tissue]]. This [[collagen]]-based material can change its mechanical properties under nervous control rather than by muscular means. This tissue enables a starfish to go from moving flexibly around the seabed to becoming rigid while prying open a [[Bivalvia|bivalve mollusc]] or preventing itself from being extracted from a crevice. Similarly, sea urchins can lock their normally mobile spines upright as a defensive mechanism when attacked.<ref>{{harvnb|Motokawa|1984}}</ref><ref>{{harvnb|Brusca|Moore|Shuster|2016|p=980}}</ref>