Hemichordates are a group of marine animals that are part of the phylum Hemichordata. They are characterized by their three-part body structure, which includes a proboscis, a collar, and a trunk. Hemichordates are primarily found in shallow waters and can be either free-swimming or burrowing organisms. This group includes two main classes: Enteropneusta, commonly known as acorn worms, and Pterobranchia, which are small, colonial animals. Hemichordates share some features with chordates, such as a notochord-like structure, which suggests a close evolutionary relationship between these groups.