Xenoturbellid

Xenoturbella
Xenoturbella japonica
Scientific classification
Kingdom:	Animalia
Phylum:	Xenacoelomorpha
Subphylum:	Xenoturbellida Westblad 1949
Family:	Xenoturbellidae Westblad 1949
Genus:	Xenoturbella Westblad 1949

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 discovered in 1915 by Sixten Bock, but it was only properly described in 1949 by Einar Westblad.

Xenoturbella has a very simple body plan. It consists of two ciliated epithelial layers: an external epidermis and an internal gastrodermis lining the simple sack-like gut. The mouth is located ventrally and there is no anus, waste is dispelled through the same opening as food is taken in.

The nervous system is composed by 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, a circulatory and an excretory system. In fact, there are no defined organs, except for a containing flagellated cells. 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.

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 water surface) and ciliated. They feature no mouth and they do not apparently feed. They are similar to the juveniles of acoelomate Neochildia fusca.

Currently the genus Xenoturbella contains 6 recognized species:

The two smaller species, X. bocki and X. hollandorum, which are up to 4 centimetres (1.6 in) long, are found in shallower waters less than 650 metres (2,130 ft) deep and form a separate clade from the other three larger species, 10 centimetres (3.9 in) or greater long, which live in deeper waters 1,700–3,700 metres (5,600–12,100 ft).

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