It is the spicules that remain in the fossil record. Filter feeding , usually sessile organisms. Over 3000 species of sponge exist today that are not much different from their Paleozoic ancestors. Modern members include jellyfish and coral. Paleozoic Cnidaria consisted of simple corals.
Corals are epifaunal, sessile filter feeders.
Colonial mass animals that look like twigs. Individual bodies or zooids are contained in calcium. Over 4000 modern members but even more abundant in the Paleozoic. Bivalves are motile filter feeders with two shells that are symmetric top to bottom but not left to right. Appear in the Cambrian but not abundant until the Late Paleozoic.