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Acorn worms are a type of aquatic invertebrate that lives in burrows on shorelines or in shallow waters. The name of the worm comes from the acorn-shaped segment at the front end of its body. There are close to 100 types of these worms, ranging in size from 1/25th of an inch to nearly 8 feet long (1 millimeter to 2.5 meters). The worms eat material that can be found in the sand in which they create their burrows or floating freely in the water around them.
The family of animals to which acorn worms belong is called the Hemichordata, and the worms themselves are also known as enteropneusts. They are very closely related to another class of sea creatures that includes sea stars, displaying a common lineage based on their distinctive anatomical features. The anatomy of the worms has led to scientists studying them as one of the linking species between vertebrates and invertebrates.
One of the defining features of acorn worms is the way their bodies are arranged. Their bodies have three sections, starting with the acorn-shaped area on the front end of the worm. This acorn is an organ used to help the worm burrow into the ground and to help draw food into the body to be processed.
The second part of the body is called the collar, and it helps to seat the acorn organ. The final area is known as the trunk. The entire body is covered in small hairs known as cilia. The acorn worms use these hairs to help them move, burrow and direct food into their mouths. As they grow, the worms continually develop gill slits in two lines along the length of their bodies; these are used mainly for breathing.
Internally, acorn worms have two hollow nerve cords. These nerve cords indicate that the worms are related to ancient vertebrates. Their development of gill slits means they also are related to a class of animals known as chordates. This combination of ancestry is studied because it could mean these worms are the link between these two types of animals.
Acorn worms spend nearly all of their time in burrows or under other protective structures such as rocks, so they are almost never seen by humans in the wild. Instead, evidence of the worms can be found in their castings. The worms raise the end of their bodies out of their burrows and release a long string of digested sand, minerals and other excrement. These castings form ribbons that remain on the bottom of the water around their burrows.