Bryozoans
Bryozoans (bry'-oh-zoh'-ahns) are sedentary, aquatic, colony-forming animals which primarily live in the sea, with a few capable of living in brackish or fresh waters.
Each colony arises from a primary zooid or ancestrula (ancestor), and all the zooids normally remain in communication throughout the colony.
Here's a drawing of a branching colony.
As you can see, this colony consists of mulltiple zooids that are in open connection with each other. The common tube forks in several places to provide room for more zooids.
Sometimes the common tube blends together and the zooids are packed close to each other or the common tube has fused – Sometime the bryozoans have a shared tube from where the zooids extend.
Here's a drawing of this type of colony:
Each zooid secretes a rigid (calcium carbonate) or gelatinous wall, which provides a living space where it can hide within as well as provide support for the colony.
When extended out from its tube (shell), each zooid has a series of slender, ciliated, post-oral tentacles. Below these “hairy” tentacles is its mouth.
It the zooid is disturbed, it retracts into the safety of its tube.
Here's a detailed drawing of its anatomy.
All of these drawings come from G. J. Allman's monograph published in 1856.
For a wonderful, in-depth discussion of Bryozoans, please refer to Michiel van der Waaij page: www.bryozoans.nl
For Michiel's picture page go here: http://www.bryozoans.nl/index.html
Fossils
Fossil bryozoans are found in rocks beginning in the Early Ordovician as part of the Ordovician radiation.
They were often major components of Ordovician seabed communities and, like modern-day bryozoans, played an important role in sediment stabilization and binding, as well as providing sources of food for other bottom-feeding organisms.
During the Mississippian (354 to 323 million years ago) bryozoans were so common that their broken skeletons form entire limestone beds. Bryozoan fossil record comprises more than 1,000 described species.
It is plausible that the Bryozoa existed in the Cambrian but were soft-bodied or not preserved for some reason; perhaps they evolved from an early worm-like ancestor at about that time.
