Enteropneusta

(invertebrate zoology) The acorn worms or tongue worms, a class of the Hemichordata; free-living solitary animals with no exoskeleton and with numerous gill slits and a straight gut.

A class of Hemichordata with approximately 70 species commonly known as acorn worms. They are free-living, solitary animals with a very soft cylindrical body lacking external appendages. The size is highly variable, from 1 in. to 8 ft (2.5 cm to 2.5 m), and the color ranges from white to shades of violet. The body is covered with cilia and mucus and is always divided into proboscis, collar, and trunk. Sometimes the animal smells like iodoform, and sometimes it shows luminescence.

The acorn worms usually live in shallow waters, buried in the sandy or muddy bottoms, but some species occur at depths of more than 9900 ft (3000 m). The proboscis and collar (the acorn) are used in excavating burrows. During this activity, water, slime, sediment, and organic particles enter the mouth and pass into the gut. Seawater is filtered and expelled through the gill slits, while the organic matter and sediment are retained to be digested.

The trunk is differentiated into four regions: the branchial region, externally recognized by two longitudinal rows of dorsal gill pores; the genital region, characterized by the gonads, which occur dorsolaterally; the hepatic region, distinguishable by a darker color and sometimes by the presence of external sacculations. The final region, the abdominal, is simply tubular with a terminal anus.

The sexes are distinct. Each gonad opens externally in a pore through which the gametes are shed. Either the eggs are relatively yolky and develop directly, with a free-swimming larval stage that hatches into a miniature acorn worm, or they lack yolk and develop through a planktonic tornaria larval stage. Only one species (Xenopleura vivipara) is viviparous, and only one (Balanoglossus capensis) reproduces asexually, but all enteropneusts have strong regeneration power. See also Hemichordata.