Editing Sea urchin (section)

=== Life cycle and development ===
[[File:Echinocardium cordatum (Pennant, 1777) early pluteus width ca.JPEG|thumb|upright=0.6|''Pluteus'' larva has [[bilateral symmetry]].]]

In most cases, the female's eggs float freely in the sea, but some species hold onto them with their spines, affording them a greater degree of protection. The unfertilized egg meets with the free-floating sperm released by males, and develops into a free-swimming [[blastula]] embryo in as few as 12 hours. Initially a simple ball of cells, the blastula soon [[gastrulation|transforms]] into a cone-shaped '''echinopluteus''' larva. In most species, this larva has 12 elongated arms lined with bands of cilia that capture food particles and transport them to the mouth. In a few species, the blastula contains supplies of nutrient [[yolk]] and lacks arms, since it has no need to feed.<ref name=IZ />

Several months are needed for the larva to complete its development, the change into the adult form beginning with the formation of test plates in a juvenile rudiment which develops on the left side of the larva, its axis being perpendicular to that of the larva. Soon, the larva sinks to the bottom and [[metamorphosis|metamorphoses]] into a juvenile urchin in as little as one hour.<ref name=Ruppert/> In some species, adults reach their maximum size in about five years.<ref name=IZ /> The [[purple urchin]] becomes sexually mature in two years and may live for twenty.<ref>{{Cite web|url=http://animaldiversity.org/accounts/Strongylocentrotus_purpuratus/|title=''Strongylocentrotus purpuratus''|website=Animal Diversity Web|year=2001|author=Worley, Alisa|access-date=2016-12-05|archive-date=2024-04-23|archive-url=https://web.archive.org/web/20240423125155/https://animaldiversity.org/accounts/strongylocentrotus_purpuratus/|url-status=live}}</ref>