GASTROTRICH

any of about 1,800 species of the class Gastrotricha, a group of microscopic aquatic invertebrates belonging to the phylum Aschelminthes. They occur in salt water and fresh water and also on sandy seashores. The body of a gastrotich, which ranges in size from 0.1 to 1.5 millimetres (0.004 to 0.06 inch), is covered by a cuticle that is often scaly and spiny. The head is swollen and lobelike. Adhesive tubules are used for anchorage. Ciliaespecially on the head and on the ventral, or bottom, surfaceare used for locomotion. Bacteria, organic debris, and certain protozoans (diatoms) are ingested by a sucking muscular pharynx, which leads to the intestine. Many species are hermaphroditic (i.e., reproductive organs of both sexes occur in the same individual). One group, Chaetonotoidea, has only parthenogenetic females (i.e., they produce unfertilized eggs that, in turn, produce females). Two types of eggs are known for freshwater gastrotrichs: tachyblastic eggs, which develop immediately, and opsiblastic eggs, which remain inactive for long periods and can survive dry and freezing conditions.