This group includes the bryozoans, the "moss animals," named that because of the encrusting form of some bryozoans, seemingly covering the shells of other organisms such as clams, like moss growing on a rock. Bryozoans are colonial. Fossil forms include solid, branching forms and other types that form an encrusting over shells of other organisms (hence, the "moss" reference). No matter what form, the individual bryozoan animals are tiny, living in a tiny spot within the calcite skeleton. And individual bryozoan colonies involve hundreds of little animals. Bryozoans have been identified as closest relatives of brachiopods (see how they are sister groups on the cladogram), because of a structure called the lophophore, present in the feeding parts of the animal.
First, let's take a look at the massive, branching (ramose = branching) forms of bryozoans from the Paleozoic Era:
That fossil might not look like much, but if we were to zoom in, we would see the fascinating scene -- each of those tiny white specks was the home of an individual bryozoan animal!
Inside each of those little holes lived a tiny bryozoan animal, looking like this:
Image Source: Wikipedia
We understand a lot about the anatomy of these tiny organisms, because we have powerful microscopes today, capable of looking a minute detail in fossils and living animals .
A special bryozoan lived during the Paleozoic Era that had delicate, wrapping, "frond-like" (or "apron-like") panels that swept in a spiral around a massive axis. This is Archimedes , named after Archimede's Screw . More often than not, the delicate panels break off, so that specimens of Archimedes usually show only the spiral axis, the more massive part of the skeleton: