Sponges are among the oldest and most primitive animals.  Fossils of sponges have been found that date back to the late precambrian period,  more than 540 million years ago.  Although sponges are primitive, they are survivors, with about 5,000 species living in the oceans today.  

     Scientists are doing DNA research on sponges, trying to determine how they are related to other animals.  It is possible that all animals are descended from one group of the sponges. 

     Sponges are communities of cells.  Adult sponges are sessile, which means that they stay in one location for their whole lives.  They attach themselves to a hard surface with a holdfast.

       Unlike other animals, sponges have no particular specific shape.  Many of them tend to be shaped somewhat like urns or vases.  Sponges have closely packed cells on the outside, with pores or small openings that admit water into the sponge. The interior cells have flagella, which create a current that draws water into the sponge.  The water has little bits of organic matter in it, and the sponge cells capture some of these bits and digest them.  The water exits through the top of the sponge as more water is drawn in. 

     This method of food capture is called filter feeding or suspension feeding because the sponges live by filtering organic material that is suspended in the water.  We will meet other filter feeders in other phyla.

     Some of the very small sponges have only two layers of cells, an inside layer and an outside layer.  

However the larger sponges have a mostly non-cellular gelatinous layer between the outside and the inside cells.  This middle layer is made of proteins and has some amoeba-like cells living in it.  These cells secrete skeletons that provide support for the sponges, and help to transport chemicals around between the other cells.

     Sponges build their skeletons from two different minerals.  These skeletons support the larger sponges.  

    One group of sponges builds its skeletons of  crystals of calcium carbonate, which is dissolved in seawater.  This mineral is used by most  organisms that  make shells or have bones. 

      Scientists studying the ocean tell us that, at a depth of 4500 meters (about 14,764 feet) calcium carbonate dissolves at about the same rate that molecules of it are floating down from above, so there isn't any calcium carbonatet here for animals  to use for building support structures such as bones and shells.  This is called the calcium carbonate compensation depth.  Did these sponges perhaps evolve in some cold, deep, calcium-poor environment?

      Another group of sponges builds its skeletons of  silica, which is glass.  Diatoms use silica to make their tiny, box-like shells, and some sponges build their skeletons out of silica, making tiny needle-like spicules in bundles.  These sponges live in deep, cold water. 

      Sponges are interesting because their structures are informal, yet their cells share some chemicals.  They do not have organs or tissues, yet the cells cooperate to some degree.

      Here is something amazing about sponges.  You take a sponge and put it through a sieve.  Leave the clumps of cells in sea water and after a while the sponge cells will reform and put the sponge together again.  Amazing!