a group of extinct invertebrates of the phylum Coelenterata that were closely related to the Hydrozoa. The organisms lived from the Ordovician to the Eocene. They formed spherical or lamellate colonies that ranged in size from a few centimeters to several decimeters and consisted of closely packed prismatic tubules 0.15–1.2 mm in diameter, with calcareous walls. The tubules had numerous horizontal partitions.
The Chaetetida consisted of approximately 15 genera. They lived in the shallows of seas and were sessile. They are of interest in studying the development of the Coelenterata.