Gastrotricha

Gastrotricha: see aschelminths.

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Gastrotricha [ga′strä·trə·kə]

(invertebrate zoology)

A group of microscopic, pseudocoelomate animals considered either to be a class of the Aschelminthes or to constitute a separate phylum.

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Gastrotricha

A phylum of minute metazoan animals (formerly placed in the aschelminth group) numbering 500 described species worldwide. Some 300 species have been reported from the marine habitat, with new ones being described every year.

Gatrotrichs comprise two orders, the Macrodasyida and the Chaetonotida. The term Gastrotricha refers to the ventral locomotor cilia by which the animals glide gracefully over the substratum or through its interstices; unlike many other ciliated animals, they cannot move in reverse. Gastrotrichs have a complete digestive tract, with a sucking pharynx, a simple intestine with a wall only a single cell thick, and an anus. They appear to be selective feeders on bacteria, very small protozoa, and yeasts. Most have protonephridia, accounting in part for their broad salinity tolerances.

Gastrotrichs appear to be regionally cosmopolitan, with 20–30% having broad distributions within continents, and 10–15% between continents; endemism probably does not exceed 20%.

The phylum Gastrotricha is the most primitive in the aschelminth group of phyla. Gastrotrichs and nematodes probably share a common ancestor, which in turn was descended from a stock that included gnathostomulids and turbellarianoid animals. See Nemata