They have a common coelom for the whole colony. Zooids are suspended in jelly. They have a sparse fossil record and uncertain ancestry. They have calcified tubular or conical zooids that continue to lengthen throughout life. Most have diaphragms.
Limited polymorphism is known.
They often lack diaphragms. They radiated in Mesozoic. Many are encrusting forms. They resemble the cyclostomes, but have cyst-like ornaments between zooecia, and lunaria around their apertures. They are stony Paleozoic forms.