The gastrotrichs (from Greek γαστήρ, gaster "stomach" and τρίξ, thrix "hair", often called hairy backs) are a phylum of microscopic (0.06-3.0 mm) animals abundant in fresh water and marine environments. Most fresh water species are part of the periphyton and benthos. Marine species are found mostly interstitially in between sediment particles, while terrestrial species live in the water films around grains of soil.
Gastrotrichs are bilaterally symmetric, with a transparent body and a flat underside. Many species have a pair of short projections at the posterior end. The body is covered with cilia, especially about the mouth and on the ventral surface, and has two terminal projections with cement glands that serve in adhesion. This is a double-gland system where one gland secretes the glue and another secretes a de-adhesive to sever the connection. Like many microscopic animals, their locomotion is primarily powered by hydrostatics.
Gastrotrichs demonstrate eutely, with development proceeding to a particular number of cells, and further growth coming only from an increase in cell size. Originally they were thought to have a body cavity (pseudocoel), but this was an artifact created by preservation methods, and they are now known to be acoelomate.
The mouth is at the anterior end, and opens into an elongated pharynx lined by myoepithelial cells. In some species, the mouth includes an eversible capsule, often bearing teeth formed from the outer cuticle of the body wall. The pharynx opens into the intestine, which is lined with glandular and digestive cells. The anus is located close to the hindmost part of the body. In some species, there are pores in the pharynx opening to the ventral surface; these may allow egestion of any excess water swallowed while feeding.
The excretory system consists of a single pair of protonephridia, which open through separate pores on the lateral underside of the animal, usually in the midsection of the body. Unusually, the protonephridia do not take the form of flame cells, but instead the excretory cells consist of a skirt surrounding a series of cytoplasmic rods that in turn enclose a central flagellum. These cells, termed cyrtocytes, connect to a single outlet cell which passes the excreted material into the protonephridial duct.
As is typical for such small animals, there are no respiratory or circulatory organs. Nitrogenous waste is probably excreted through the body wall, as part of respiration, and the protonephridia are believed to function mainly in osmoregulation.
The nervous system is relatively simple. The brain consists of two ganglia, one on either side of the pharynx, connected by a commisure. Each ganglion gives rise to a single nerve cord, which runs the length of the body and includes further, smaller ganglia.
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