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(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 marine animals of phylum Hemichordata.

Enteropneusts are from several cm to 1 m in length. The body is wormlike and consists of three sections: proboscis, collar, and trunk. An oral opening at the ventral base of the proboscis leads to the pharynx. The side walls of the gullet are perforated by two rows of gill slits. Tiny food particles, entering the gullet with water, are cemented by mucus into clumps, which in turn are moved by cilia toward the gut. The gut ends in an anal opening at the posterior end of the body.

The excretory organ is a plicate section of the wall of the proboscis called the glomerulus, and decomposition products are excreted through a pore in the proboscis. The nervous system consists of two trunks (dorsal and ventral) and of plexuses of nerve cells lying under the ectoderm. The cardiovascular system consists of dorsal and ventral vessels.

The Enteropneusta are dioecious: the gonads (more than 30 pairs) are arranged along the gut, their ducts opening to the sides of the body. Cleavage of the fertilized egg is holoblastic and almost equal. As in other deuterostomes, the anal opening forms at the site of the primitive mouth of the gastrula; the mouth opening is formed later. The gastrula is transformed into a tornaría, which is very similar to larva of echinoderms. The Enteropneusta have a secondary body cavity that is formed by the joining from the midgut of five coelomic sacs, or coeloms.

The Enteropneusta number about 100 species. They are found mostly in warm seas. In the Soviet Union, one species is found in the White Sea, one in the Barents Sea, and several in the Bering, Okhotsk, and Japan seas. They maintain a fossorial mode of life on the sea floor, feeding on decomposing animal and plant residues. Russian scientists such as A. O. Kovalevskii, E. Metchnikoff, and V. M. Shimkevich, have played an outstanding role in studying the structure, development, and philogeny of the Enteropneusta. Balanoglossus is a typical representative of the class.


Dogel’, V. A. Zoologiia bespozvonochnykh, 5th ed. Moscow, 1959.