Sponge

Sponges were some of the very first invertebrates, known at least since the Cambrian period 550 MYA, but reaching their peak in the Mesozoic (though Walking with Dinosaurs ep. 3 does not show this). 

Facts

They were some of the first multicellular invertebrates, similar to some of their contemporaries, like the corals and sea anemones in living a sedimentary lifestyle and having a circular (geometrically speaking) body plan, dominated by a very primitive digestive system, but unlike them, they lacked the stinging cells, and their free-floating plankton phase was more brief. it can grow itself if it is smashed into tiny pieces.

In Walking with... Series

In Walking with Monsters ep.1 the sponges were shown as some typical marine invertebrates of the Silurian era alongside the sea urchins and the now-extinct giant orthocone. Although it is technically correct, true coral has appeared by the Ordovician and began to form the first 'true' coral reefs, thus the sponges weren't as dominant as the show makes them appear to be.