Sponges, or Porifera, are simple invertebrates and filter feeders, meaning they collect small particles of food that are floating in the water. What is most interesting about sponges is their level of organization. Sponges are the only multicellular animals that are organized at the cellular level. This means that they function without forming discreet tissues and organs. Instead, the whole mass of the sponge is penetrated by innumerable tubes and chambers, (the aquiferous system). Water flows through this system as a result of the actions of specialized cells, called choanocytes. One of the biggest advantages of this system is that any cell can quickly specialize to do any function. This allows the sponge to have great powers of regeneration.