The first Multicellular Organisms?
Increased co-ordination between colonial cells appeared with the evolution of the sponges (Porifera). Sponges may be formless lumps on the sea floor reaching two metres in size. Their surfaces are covered with tiny pores through which water is drawn into the body by flagella and then expelled through larger vents. The sponges feed by filtering particles from this stream of water passing through its body. Some sponges produce a soft flexible silica-based substance which supports the whole organism, whereas other sponges secrete lime or silica to create a hard "skeleton" for support. Despite the elaborate skeletons that some sponges are able to produce they cannot be considered as an integrated multi-cellular animals since they have no nervous systems nor muscle fibres.
The structure of a sponge is simple: it is shaped like a tube, with one end stuck to a rock or other object and an open end, the osculum, open to the environment. The spongocoel, or interior of the sponge, is composed of walls perforated with microscopic pores that allow water to flow through the spongocoel.