Xyloplax medusiformis

Xyloplax medusiformis
Scientific classification
Kingdom:	Animalia
Phylum:	Echinodermata
Class:	Asteroidea
Order:	Peripodida
Family:	Xyloplacidae
Genus:	Xyloplax
Species:	X. medusiformis
Binomial name
Xyloplax medusiformis Baker, Rowe & Clark, 1986

Xyloplax medusiformis is a sea daisy, a member of an unusual group of marine taxa belonging to the phylum Echinodermata. It is found at bathyal depths in waters around New Zealand. It was first described in 1986 by Baker, Rowe and Clark and is the type taxon of the genus Xyloplax. Its generic name derives from the Greek "xylo" meaning wood and its specific name was chosen because its morphology superficially resembles that of a cnidarian medusa.

Specimens of Xyloplax medusiformis were first discovered by accident when a submersible craft was being used to collect samples of wood on the deep sea bed in the South Pacific near New Zealand. Nine individuals were discovered from five locations. At the time these specimens puzzled researchers because of their lack of close affiliations to other echinoderms. Since then further members of the genus Xyloplax have been discovered on wood at bathyal depths, Xyloplax turnerae in the Atlantic Ocean off the Bahamas and Xyloplax janetae in the central Pacific Ocean.

Xyloplax medusiformis was the first sea daisy to be described. It was clearly an echinoderm but at first it was considered to be sufficiently different from starfish, class Asteroidea, as to warrant it being placed in a new class of its own, the Concentricycloidea. The main difference was that the water vascular system of asteroids has a single ring canal circling the mouth connected to radial arms with short lateral side arms while that of the sea daisy had two ring canals surrounding the mouth, linked by five inter-ring canals. The concentric arrangement of plates on the surface of the sea daisy was novel and it had a single row of podia (tube feet) rather than the two rows typical of asteroids.

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