Ambulacraria

Ambulacraria, from the latin ambulacrum, meaning "a walk planted with trees", was named by Élie Metchnikoff in reference to the larval similarities including a folded band lined with cillia that are used for both feeding and locomotion. Metchnikoff also noted similarities among the coelomic systems of echinoderms and hemichordates and their larvae. ==Phylogeny==

The two living clades with representative organisms are: • Echinodermata (sea stars, sea urchins, brittle stars, sea cucumbers, feather stars, sea lilies, etc.) • Hemichordata (acorn worms (Enteropneusta) and Pterobranchia (including Graptolithina)) (These together sometimes are called the lower deuterostomes. A third, extinct clade known as the Cambroernida has recently been recognized as part of the ambulacrarian stem group. The morphology of this group suggests that the ancestral deuterostome did not possess a post-anal tail, which evolved convergently in chordates and hemichordates, and possessed gill slits but not additional structures such as the gill bars found in hemichordates. or stem-group ambulacrarians. Vetulocystids have also been tentatively proposed as the basal-most stem-group chordates, while Yanjiahella has also been proposed to be a stem-group hemichordate. with the possible placements of uncertain potential ambulacrarians shown with dashed lines and question marks: }} ==Ontogeny==

As for many animals, the egg cell of any extant ambulacrarian divides and develops into a blastula ("cell ball"), which develops into a triploblast ("three-layered") gastrula. The gastrula then develops into a dipleurula larva form in the Asteroidea, Holothuroidea, Crinoidea, and Hemichordata, and into a pluteus larva form in the Echinoidea and Ophiuroidea. This, in its turn, is developed in various different kinds of larvae for different taxa of ambulacrarians. It has been suggested that the adult form of the last common ancestor of the ambulacrarians was anatomically similar to the dipleurula larva; this hypothetic ancestor sometimes also is called dipleurula. == References ==