The Hemichordates, or Acorn Worms and Pterobranchs are a small phylum of worm-shaped marine animals that were once considered to be a part of the Chordates but are now considered to be related to the Echinoderms. The larvae of some hemichordates look very much like those of some echinoderms. There are up to 300 species of Hemichordates.
The bodies of Hemichordates are divided into three parts - proboscis, collar and trunk. They have open circulatory systems and a complete digestive tract but the muscles in their digestive tract are very poorly developed, and food moves through it by using the cilia that cover its insides. They have "gill slits" that open into the pharynx.
They vary widely in size, from up to 2.5 m for the around 90 species of acorn worms to up tp 1 mm for the pterobranchs. These two groups also vary in appearance and behaviour.