Eleutherozoa

Eleutherozoa is a proposed subphylum of echinoderms. They are mobile animals with the mouth directed towards the substrate. They usually have a madreporite, tube feet, and moveable spines of some sort, and some have Tiedemann's bodies on the ring canal. All living echinoderms except Crinoidea belong here. There are 2 main competing hypotheses about the internal subdivision, both about equally well supported by both molecular and morphological data. They differ in their placement of the Ophiuroidea (brittle stars), and are named accordingly. The 'Cryptosyringida' hypothesis posits that the 'sea-star' morphology is plesiomorphic for Eleutherozoa as a whole, and that starfish (Asteroidea) and brittle stars are not very closely related, the latter forming the clade Cryptosyringida together with the Echinozoa. The 'Asterozoa' hypothesis, on the other hand, implies that the 'sea-star' arms of starfish and brittle stars, as well as the rounded shape of Echinozoa, all evolved independently from an ancestor of unknown morphology, but that each 'armed' and 'rounded' lineage is strictly monophyletic. Too little is known of the basal eleutherozoans and echinoderms to be able to firmly decide for or against any of these hypotheses at present.