Sponges are a primitive form of multi-cellular animal.
They present the remarkable ability of re-assembling themselves into the original form after being broken into small fragments. It is this ability that suggests a link to the process of differentiation of cells in higher organisms. Sponges have been touted as a possible long term ancestor of all vertebrates. A wide variety of forms have been present from the earliest Cambrian to the present day. Only calcareous sponges are preserved well in the fossil record and as classification is chiefly by soft parts they are of limited stratigraphical value.