Acoelomorpha and xenoturbellida

Abstract

Acoelomorpha, comprising Acoela and Nemertodermatida, and Xenoturbellida (with one single hitherto described species, Xenoturbella bocki) are simple, aquatic, acoelomate worms that measure between 100 μm and 1 cm. Acoelomorpha and Xenoturbella are found to cluster together as the monophyletic Xenacoelomorpha in some recent molecular phylogenetic analyses. With only few exceptions, all species are marine, with most of them living in the interstitial environment. Xenoturbellids and acoelomorphs possess a simple nervous system that generally is a basiepidermal nerve net; however, in some cases this net is condensed into basiepidermal neurite bundles at different parts of the body or is submerged under the epidermis where condensed brains and submuscular cords are formed. Some Acoela possess eye spots, while most nemertodermatids, Xenoturbella, and Acoela lack eyes. Recent internal phylogenetic analyses suggest that eyes were absent from the ground pattern of Acoelomorpha. A prominent gravitational sensory organ, the statocyst, is present in all xenacoelomorph taxa, albeit with differing ultrastructure.