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.

The first known species (Xenoturbella bocki) was collected in 1878 and 1879 in the Gullmar fiord on the Swedish west coast by August Malm and is stored in the collection of the Gothenburg Natural History Museum. A specimen is on display in the exhibition. It was collected again in the Gullmar fiord in 1915 by Sixten Bock, but it was only properly described in 1949 by Einar Westblad. The type specimens are kept at the Swedish Museum of Natural History in Stockholm.

Xenoturbella has a very simple body plan. It consists of a dorsoventrally flattened acoelomate body, with a ventral furrow on each side running down from the anterior tip until it is stopped by an anterior circumferential furrow. It shows two ciliated epithelial layers: an external epidermis and an internal gastrodermis lining the simple sac-like gut. The epidermis and gastrodermis are separated by a thick and multilayered basement membrane called the "subepidermal membrane complex", a major part of the extracellular matrix. The multiciliate epidermis displays unique interconnected ciliary rootlets and a mode of withdrawal and resorption of worn epidermal cells. The mouth is a mid-ventral pore leading to a gastral cavity, and there is no anus: waste is dispelled through the same opening as food is taken in.

The nervous system is composed of a net of interconnected neurons beneath the epidermis, without any concentration of neurons forming ganglia or nerve cords.

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. 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.

Research on the species Xenoturbella bocki has shown it to have external fertilization, with eggs and sperm being released from new openings in the body wall. Gametes released into the water through ruptures also occur in Xenoturbella's closest relatives, the acoels and nemertodermatids. No examples of hermaphroditism were reported.

Eggs of Xenoturbella are 0.2 millimetres (0.0079 in) wide, pale orange, and opaque. Newly hatched embryos are free-swimming (tending to stay close to the water surface) and ciliated. They feature no mouth, and they do not apparently feed. They are similar to the juveniles of the acoelomate Neochildia fusca.

The term Xenoturbella derives from the Ancient Greek word ξένος (xénos), meaning "strange, unusual", and from the Latin word turbella meaning "stir, bustle". This refers to the enigmatic, unusual taxonomic status of the animal, initially considered as related to turbellarians, a group of flatworms whose aquatic species stir microscopic particles close to their ciliated epidermis.