The coelenterates are the simplest organisms that have three distinct cellular layers, typical of all more complex animals. The coelenterates called corals are abundant in the geologic record. Brachiopods were abundant in the Paleozoic and still exist in a few places. They superficially look like clams but are completely different in their internal organs. Brachiopods have symmetrical shells but the two mating shells are different.
Mollusks are abundant fossils.
During the Paleozoic, cephalopods , relatives of the nautilus, squid and octopus, were abundant. Some had coiled shells like a nautilus, others had long straight conical shells. The most abundant organisms in terms of number of species are the arthropods, including insects, spiders, scorpions, and crustaceans. Most of the fossils shown here are trilobites, which were abundant throughout the Paleozoic but then became extinct. Echinoderms include starfish, sand dollars and sea urchins.