ACORN WORM

also called Enteropneust, any of the soft-bodied invertebrates of the class Enteropneusta, phylum Hemichordata. The front end of these animals is shaped like an acorn, hence their common name. The "acorn" consists of a proboscis and a collar that may be used to burrow into soft sand or mud. The animals vary in length from about 5 cm (about 2 inches) in certain Saccoglossus species to more than 180 cm (about 6 feet) in Balanoglossus gigas. Acorn worms live along the seashore and in water to depths of more than 3,200 m (10,500 feet). Most live in U-shaped burrows, but some deepwater species swim freely over the bottom. They filter food from seawater that passes into the mouth and out through the gill slits. Some species secrete a slime that is swept into the mouth by cilia, or tiny hairs, carrying food particles with it. Paired gonads are located next to the gills, which lie behind the collar. Females of some species lay a few large eggs with much yolk; others lay many small eggs with little yolk. Some hatch into miniature acorn worms; others hatch into an immature form called tornaria larva.