Editing Echinoderm (section)

===Larval development===
[[File:Pluteus001.jpg|thumb|A [[bilaterally symmetric]] echino[[pluteus larva]] with larval arms]]

Development begins with a bilaterally symmetrical embryo, with a coeloblastula developing first. [[Gastrulation]] marks the opening of the "second mouth" that places echinoderms within the deuterostomes, and the mesoderm, which will host the skeleton, migrates inwards. The secondary body cavity, the coelom, forms by the partitioning of three body cavities. The larvae are often [[plankton]]ic, but in some species the eggs are retained inside the female, while in some the female broods the larvae.<ref>{{harvnb|Dorit|Walker|Barnes|1991|p=778}}</ref><ref>{{harvnb|Brusca|Moore|Shuster|2016|pp=997–998}}</ref>

The larvae pass through several stages, which have specific names derived from the taxonomic names of the adults or from their appearance. For example, a sea urchin has an 'echinopluteus' larva while a brittle star has an 'ophiopluteus' larva. A starfish has a '[[bipinnaria]]' larva, which develops into a multi-armed '[[brachiolaria]]' larva. A sea cucumber's larva is an 'auricularia' while a crinoid's is a 'vitellaria'. All these larvae are [[bilaterally symmetrical]] and have bands of cilia with which they swim; some, usually known as 'pluteus' larvae, have arms. When fully developed, they settle on the seabed to undergo metamorphosis, and the larval arms and gut degenerate. The left-hand side of the larva develops into the oral surface of the juvenile, while the right side becomes the aboral surface. At this stage, the pentaradial symmetry develops.<ref>{{harvnb|Dorit|Walker|Barnes|1991|p=778}}</ref><ref>{{harvnb|van Egmond|2000}}</ref>

A [[planktotrophic|plankton-eating]] larva, living and feeding in the water column, is considered to be the ancestral larval type for echinoderms, but in extant echinoderms, some 68% of species develop using a [[lecithotrophic|yolk-feeding]] larva.<ref name=Uthicke/> The provision of a yolk-sac means that smaller numbers of eggs are produced, the larvae have a shorter development period and a smaller dispersal potential, but a greater chance of survival.<ref name=Uthicke/>