Editing Sea urchin (section)

=== Reproduction ===
[[File:Male Flower Sea Urchin (toxopneustes roseus).theora.ogv|thumb|Male flower urchin (''[[Toxopneustes roseus]]'') releasing milt, November 1, 2011, Lalo Cove, Sea of Cortez]]

Sea urchins are [[dioecious]], having separate male and female sexes, although no distinguishing features are visible externally. In addition to their role in reproduction, the [[gonad]]s are also nutrient storing organs, and are made up of two main type of cells: [[germ cell]]s, and [[somatic cell]]s called nutritive phagocytes.<ref>{{Cite journal |pmc = 5090362|year = 2016|last1 = Gaitán-Espitia|first1 = J. D.|title = Functional insights into the testis transcriptome of the edible sea urchin Loxechinus albus|journal = Scientific Reports|volume = 6|pages = 36516|last2 = Sánchez|first2 = R.|last3 = Bruning|first3 = P.|last4 = Cárdenas|first4 = L.|pmid = 27805042|doi = 10.1038/srep36516|bibcode = 2016NatSR...636516G}}</ref> Regular sea urchins have five gonads, lying underneath the interambulacral regions of the test, while the irregular forms mostly have four, with the hindmost gonad being absent; heart urchins have three or two. Each gonad has a single duct rising from the upper pole to open at a [[gonopore]] lying in one of the genital plates surrounding the anus. Some burrowing sand dollars have an elongated papilla that enables the liberation of gametes above the surface of the sediment.<ref name=Ruppert/> The gonads are lined with muscles underneath the peritoneum, and these allow the animal to squeeze its [[gamete]]s through the duct and into the surrounding sea water, where [[fertilization]] takes place.<ref name=IZ />