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Gastrotricha

Gastrotrichs are small, about 0.5mm in length, and are related to nematodes in that they are cycloneuralians but they do not molt their body covering. They also have the unusual innervation exhibited by nematodes in which processes extend from muscle cells to motor axons. There are about 400 species known to science.

They were once put in a phylum with rotifers, but have a thick cuticle. They lack the rotifer corona and mastax. They have a muscular pharynx and feed on a variety of living and dead organic matter, anything small enough to be swept into their mouths by the 4 tufts of beating cilia on the head. Although they have circular and longitudinal muscles they use cilia to move. The muscles allow them to flex their bodies. They have adhesive glands that they can use to temporarily attach to the substratum. They also like flatworms have glands that secrete enzyme for detachment. Like rotifers and nematodes, they exhibit eutely. Epidermis may be partially syncytial.

Gastrotrichs have a small brain and a pair of longitudinal nerves as well as various ciliary sense organs. A few species have protonephridia but otherwise they have no excretory gaseous exchange or circulatory systems. They are considered pseudocoelomates or acoelomates.

Gastrotrichs are either hermaphroditic or parthenogenetic. Most marine species are hermaphroditic possessing both male and female gonads, though only one set is functional at a time, so an individual is either functionally a female or functionally a male. Sperm is transferred from a functionally male Gastrotrich to a functionally female one via a spermatophore. Only a small number of eggs are produced at any one time and the young hatch out as small gastrotrichs, there is no larval stage. There are two types of eggs produced, one that develops immediately and another thick-shelled resting egg that is very resistant to freezing adn drying. The young feed and grow quickly and may reach sexual maturity in as little as 2 days.