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Acorn worm is the name for many similar animals of the phylum Hemichordata. Within its natural habitat an acorn worm appears as a bright orange or red acorn sticking out of the surface of muddy or sandy shores. When carefully dug out, a fragile earthworm like animal is revealed. The length will vary from 2 inches to 6 feet depending on the species. The acorn part of this animal is the proboscis, which is attached to the body by a stalk. Surrounding the stalk in the front of the body is a cylindrical collar. The acorn worm's mouth, which is covered by this collar, opens into a straight intestine running the entire length of the body. Directly behind the collar are two rows of gill slits that connect the intestine with the exterior of its body. It has a very simple heart that pumps blood first through a kidney and then on to the intestine and gills. These animals have no sense organs, but instead simple sensory cells that are imbedded in the skin.

Most species of the acorn worm can be found from the shoreline to the depths of the ocean, even down two miles or more. They live in U or V shaped tunnels in the seabed, while some construct tubes of mud or sand particles that are glued together with slime. Others that live in deeper waters are known to move freely over the bottom. When the acorn worm moves, its movements are effected by the proboscis and collar. These water filled bags surrounded by muscles will contract to elongate the proboscis, thus forcing the animal forward. Minute whip like protoplasmic hairs pump water through openings in the walls of the proboscis and collar which causes them to swell. This swelling anchors the front part of the animal while the rest is dragged forward by muscle contractions. While the acorn worm moves around mud and sand is forced into its mouth. Water is filtered out through the gill slits and solid materials are passed down where any organic matter is digested. Undigested sand is bound in mucus and ejected.

Reproductive organs of the acorn worm lie in pairs beside the gills. Eggs are laid along the same sides of the parent's tunnor or directly into the surrounding water. Species living in deep or cold water will lay a few large eggs with a rich yolk that develop directly into baby acorn worms. Others that live in warm or shallow water lay a large number of small eggs that develop into larvae that swim on the surface of the water before settling to the bottom to become adult and worm like. On the surface acorn worms look a lot like earthworms, but the internal structure of their body sets them apart. There are certain features that give these animals an apparent affinity with back boned animals. These features include the structure of the nervous system, the rows of gill slits and a notochord. These features have caused the acorn worm to be linked with other groups of invertebrates such as the sea cucumber, relatives of the starfish and sea urchins. In the second half of the 19th century and attempt was made to bridge the gap between vertebrates and invertebrates. The discovery of a number of animals, including the acorn worm, provided the link. The acorn worm was first discovered by a Neapolitan fisherman who found fragments of the strange animal in his net and took them to a zoologist in Naples. After a careful study this zoologist was able to recognize that this one once of the missing links.