Xenoturbella
Xenoturbella is a genus of very simple bilaterians up to a few centimeters long. It contains a small number of marine benthic worm-like species.[3]
The first known species (Xenoturbella bocki) was discovered in 1915 by Sixten Bock, but it was only properly described in 1949 by Einar Westblad.[4]
Xenoturbella has a very simple body plan. It consists of dorsoventrally flattened acoelomate animals, with an anterior circumferential furrow.[5] It shows two ciliated epithelial layers: an external epidermis and an internal gastrodermis lining the simple sac-like gut. The multiciliated epiderm displays unique interconnected ciliary rootlets and mode of withdrawal and resorption of worn epidermal cells.[5] The mouth is a mid-ventral pore leading to a gastral cavity, and there is no anus:[6][5] waste is dispelled through the same opening as food is taken in.[7]
The nervous system is composed by a net of interconnected neurons beneath the epidermis, without any concentration of neurons forming ganglia or nerve cords.[8][9]
Species of Xenoturbella also lack a respiratory, circulatory and excretory system. In fact, there are no defined organs, except for an anterior statocyst containing flagellated cells and a frontal pore organ.[6][5] There are no organized gonads, but gametes are produced. Adults producing sperm are very rarely observed, but eggs and embryos are known to occur in follicles.[10]
Eggs of Xenoturbella are 0.2 millimetres (0.0079 in) wide, pale orange and opaque.[11] Newly hatched embryos are free-swimming (tending to stay close to water surface) and ciliated. They feature no mouth and they do not apparently feed.[11] They are similar to the juveniles of acoelomate Neochildia fusca.[11]
