Editing Echiura (section)

==Reproduction==
Echiurans are [[dioecious]], with separate male and female individuals. The [[gonad]]s are associated with the [[peritoneum|peritoneal]] membrane lining the body cavity, into which they release the [[gamete]]s. The sperm and eggs complete their maturation in the body cavity, before being stored in genital sacs, which are specialised metanephridia. At spawning time, the genital sacs contract and the gametes are squeezed into the water column through pores on the worm's ventral surface. Fertilization is external.<ref name=IZ/>

Fertilization is internal in the sexual dimorphic order Bonelliida, which has dwarf males living inside the female. Members of the order Echiurida have external fertilization and are sexual monomorphic.<ref>{{cite journal | url=https://www.sciencedirect.com/science/article/abs/pii/S1055790316000713 | doi=10.1016/j.ympev.2016.03.003 | title=A comprehensive molecular phylogeny of spoon worms (Echiura, Annelida): Implications for morphological evolution, the origin of dwarf males, and habitat shifts | journal=Molecular Phylogenetics and Evolution | date=June 2016 | volume=99 | pages=247–260 | last1=Goto | first1=Ryutaro | pmid=26975690 | bibcode=2016MolPE..99..247G }}</ref><ref>{{cite journal | pmc=3572977 | date=2013 | last1=Goto | first1=R. | last2=Okamoto | first2=T. | last3=Ishikawa | first3=H. | last4=Hamamura | first4=Y. | last5=Kato | first5=M. | title=Molecular Phylogeny of Echiuran Worms (Phylum: Annelida) Reveals Evolutionary Pattern of Feeding Mode and Sexual Dimorphism | journal=PLOS ONE | volume=8 | issue=2 | pages=e56809 | doi=10.1371/journal.pone.0056809 | doi-access=free | pmid=23457618 | bibcode=2013PLoSO...856809G }}</ref>

Fertilized eggs hatch into free-swimming [[trochophore]] larvae. In some species, the larva briefly develops a segmented body before transforming into the adult body plan, supporting the theory that echiurans evolved from segmented ancestors resembling more typical annelids.<ref name=IZ/>

The species ''[[Bonellia viridis]]'', also remarkable for the possible [[antibiotic]] properties of [[bonellin]], the green chemical in its skin, is unusual for its extreme [[sexual dimorphism]]. Females are typically {{convert|15|cm|in|0|abbr=on}} in body length, excluding the proboscis, but the males are only {{convert|1|to|3|mm|in|2|abbr=on}} long, and spend their adult lives within the uterus of the female.<ref name=IZ/>