Xenoturbella bocki
| Xenoturbella bocki | |
|---|---|
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| Diagram of a longitudinal section of X. bocki in Russian text | |
| Scientific classification | |
| Kingdom: | Animalia |
| Phylum: | Xenacoelomorpha |
| Family: | Xenoturbellidae |
| Genus: | Xenoturbella |
| Species: | X. bocki |
| Binomial name | |
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Xenoturbella bocki Westblad, 1949 |
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Xenoturbella bocki is a marine benthic worm-like species from the genus Xenoturbella. It was the first species in the genus discovered. Initially it was collected by Sixten Bock in 1915, but it was only described in 1949 by Einar Westblad.
Xenoturbella westbladi is a synonym for this species, given by Israelsson in 1999. It is a member of sub-phylum Xenoturbellida, which are known as paradoxmaskar - Swedish for "paradox worm" (a term that some popular media have applied to the species), due to the possibility that as a deuterostome, it may be more closely related to humans than other, more complex invertebrates such as lobsters.
This animal usually grows to one cm in length, though individuals as long as four centimeters have been reported. Its nervous system consists of a nerve net with no defined brain or ganglia. This animal lacks an anus, excreting waste through the same opening as it intakes food. The animal moves through the water via rhythmic muscle contraction, aided by the motion of numerous tiny cilia on its sides, and a tuft of longer cilia on its back. Small cells contain vesicles which may act as glands. Adults are known to have a symbiotic relationship with bacterial species. Its diet includes bivalve mollusks.
This species has been found in ocean habitats off the coast of Europe.
X. bocki lays small, mucus-coated eggs, which sink in the water column. The eggs have a pale-orange color, and are opaque. Young, upon hatching, are yellowish, nearly spherical, and move to the surface of the water. Larvae lack a blastopore and do not feed until they are fully developed. They may derive nourishment from the yolk which would make them lecithotrophic. Within five days muscular contractions are observed in a laboratory setting, which may aid locomotion. As of 2013, this animal is extremely challenging to grow in captivity.
Above the genus level, the classification of this animal is controversial. Early classifications placed it within protostoma, grouped with the bivalves. Today, this is understood to have come from misclassification due to contaminating DNA from its food. This animal, its close relatives and the Acoelomorpha are sometimes placed within the deuterostomes, while others classify these organisms as a basal offshoot that resembles a common ancestor of deuterostomes (a large clade that includes humans and other chordates, sea stars and others) and protostoma (a large clade including worms, mollusks and arthropods). Recent studies support the later (outgroup basal to both clades) interpretation, and suggest that, like Xenoturbella bocki, the common ancestor likely had one opening, ciliated locomotion and a wormlike body. In contrast, if the deuterostome hypothesis is correct, then Xenoturbella must have lost many ancestral traits, such as an anus.
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