The orifice of the zooecium may or may not have an operculum, or hatch-like lid. A colony of more than one zooecium is called a zoarium . Zoaria may occur in the shape of mats, crusts, lobes, lumps, trees, or funnels up to several centimeters in size. There is a division of functions among the zooids in a zoarium, and the zooids are differentiated accordingly. The chief distinguishing character of the bryozoa is their U-shaped digestive tract, with a mouth, esophagus, stomach, intestine, and anus.
Bryozoans reproduce both sexually and asexually.
A larva hatches from a fertilized egg and metamorphoses into the primary zooid, or ancestrula. The ancestrula then produces one or more further zooids by budding. In this manner, the zoarium is created. Bryozoans are mostly marine, but some species are estuarine or freshwater. Bryozoa are the most abundant fossils in many marine limestones, calcareous shales, and mudstones.