Hemichordata, Acanthograptus

• Graptolites such as Acanthograptus are actually colonies of tiny worm-like animals that are connected together by stem-like stolons.

• The colony is in the form of a tube with many small branches, each of which houses one individual.

• In Acanthograptus fossils, the tubes have collapsed, producing the appearance of a toothed saw.

• Like most graptolites, Acanthograptus floated in the water and collected small food particles using their tentacles.

• About twenty species are living today, and are known as pterobranchs.