Editing Sand dollar (section)

==Description==
[[File:Leodia sexiesperforata derivada 2013.jpg|thumb|''[[Leodia sexiesperforata]]'' by [[Louis Agassiz]] (1841)]]
[[File:Rotulidae.JPG|thumb|Examples of [[Rotulidae]]]]
[[Image:Encope emarginata (Leske, 1778) derivate 2013.jpg|thumb|''[[Encope emarginata]]'' (aboral and oral faces) by [[Ernst Haeckel]] (1904) ]]
[[Image:Clypeaster rosaceus (Linnaeus, 1758) derivate 2013.jpg|thumb|''[[Clypeaster rosaceus]]'' (aboral and oral faces) by [[Ernst Haeckel]] (1904)]]

Sand dollars diverged from the other irregular echinoids, namely the [[Cassiduloida|cassiduloids]], during the early [[Jurassic]],<ref>{{cite journal |last1=Kier |first1=Porter M. |title=Rapid evolution in echinoids |journal=Palaeontology |date=January 1982 |volume=25 |issue=1 |pages=1–9 |id={{BHL page|49707713}} |url=https://www.palass.org/publications/palaeontology-journal/archive/25/1/article_pp1-9 }}</ref> with the first true sand dollar genus, ''[[Togocyamus]]'', arising during the [[Paleocene]]. Soon after ''Togocyamus'', more modern-looking groups emerged during the [[Eocene]].<ref name="TPBDB"/>

Sand dollars are small in size, averaging from 80 to 100 mm (3 to 4 inches).<ref name=":0" /> As with all members of the order Clypeasteroida, they possess a rigid [[skeleton]] called a [[test (biology)|test]]. The test consists of calcium carbonate plates arranged in a fivefold [[Symmetry in biology|symmetric]] pattern.<ref>{{cite web | title = Sand Dollar Printout - Enchanted Learning Software | url = http://www.enchantedlearning.com/subjects/invertebrates/echinoderm/Sanddollarprintout.shtml | publisher = Enchanted Learning | year = 2000}}</ref> The tests of certain species of sand dollar have slits called lunules that can help the animal stay embedded in the sand to stop it from being swept away by an ocean wave.<ref name="Grzimek">{{Cite book|last=Grzimek|first=Bernhard |title=Grzimek's animal life encyclopedia |orig-date=2003|year=2004|publisher=Gale|others=Neil Schlager, Donna Olendorf, American Zoo and Aquarium Association|isbn=0-7876-5362-4|edition=2nd|location=Detroit|oclc=49260053}}{{pn|date=April 2024}}</ref> In living individuals, the test is covered by a skin of [[velvet]]-textured spines which are covered with very small hairs ([[cilia]]). Coordinated movements of the spines enable sand dollars to move across the seabed. The velvety spines of live sand dollars appear in a variety of colors—green, blue, violet, or purple—depending on the species. Individuals which are very recently dead or dying (moribund) are sometimes found on beaches with much of the external morphology still intact. Dead individuals are commonly found with their empty test devoid of all surface material and bleached white by sunlight.

The bodies of adult sand dollars, like those of other [[echinoid]]s, display [[radial symmetry]]. The petal-like pattern in sand dollars consists of five paired rows of pores. The pores are perforations in the [[endoskeleton]] through which podia for [[gas exchange]] project from the body. The mouth of the sand dollar is located on the bottom of its body at the center of the petal-like pattern. Unlike other urchins, the bodies of sand dollars also display secondary front-to-back [[bilateral symmetry]] with no morphological distinguishing features between males and females. The anus of sand dollars is located at the back rather than at the top as in most urchins, with many more bilateral features appearing in some species. These result from the [[natural selection|adaptation]] of sand dollars, in the course of their [[biological evolution|evolution]], from creatures that originally lived their lives on top of the seabed ([[Benthos#Epibenthos|epibenthos]]) to creatures that burrow beneath it ([[Benthos#Endobenthos|endobenthos]]).

<gallery style="text-align:center;" mode="packed">
Echinocyamus_pusillus.jpg|''[[Echinocyamus pusillus]]'' alive.
Live Sand Dollar trying to bury itself in beach sand.jpg|Living sand dollar.
Group of Sand dollars in Monterey Bay Aquarium.jpg|[[Eccentric sand dollar]]s (''Dendraster excentricus'') at [[Monterey Bay Aquarium]].
Echinarachnius parma (6806969731).jpg|''[[Echinarachnius parma]]'' (family [[Echinarachniidae]]).
File:Keyhole sand dollar 01.jpg|''[[Mellita quinquiesperforata]]'' test ([[Clypeasteridae]])
File:Clypeaster reticulatus both sides.jpg|''[[Clypeaster reticulatus]]'' test ([[Clypeasteridae]])
File:Echinodiscus2.jpg|''[[Echinodiscus tenuissimus]]'' test ([[Astriclypeidae]])
File:Clypeaster aegypticus (inside).JPG|''[[Clypeaster aegypticus]]'', showing internal buttresses
</gallery>