Worms and higher metazoa have approximately 55 specialized cells. Sponge image courtesy of Smithsonian Institute Sponges Sponges are the simplest grade of multi-celled animals.
Water is pulled through the body, and food is filtered out.
The cnidarians include corals, sea anemones, and jellyfish. Their basic body plan is also a sack-like form, but at one end there is a mouth which can be opened and closed, and tentacles which direct food to the mouth. Worms have a fluid-filled cavity called a coelom inside the body and variations of this cavity can be seen in all higher animals. In many animals, it has become the sack which holds the internal organs. It was originally thought that these early animals would not leave a fossil record at all because soft parts of animals are only very rarely fossilized.