The organisms within the Porifera phylum are commonly referred to as Sponges and represent the simplest animal. They are primitive, sessile, mostly marine, water dwelling filter feeders that pump water through their bodies to filter out particles of food matter. They also have no true tissues, lacking muscles, nerves, and internal organs.
Sponges are primative multicellular animals with a layer of unique collar cells, partially lining the internal water filled system of canals. These canals are used for feeding, breathing and excretion. Motile amoeboid secretory cells, such as spicule secreting, collagen secreting and feeding / transport archaeocytes lie within an interior matrix. Sponge cells do not form tissues, nor are they organised into organs for particular functions, but into two classes the cells are arranged in two layers. 
Many sponges are morphologically plastic, their growth form and colour sometimes being affected by local conditions and habitats. In some cases there are substantial morphological and biochemical differences between individuals with only small changes in environmental conditions. 
Sponges can actually move, they are not permanently fixed to a location. This is achieved via two mechanisms, physically moving the entire colony using podia and rearranging the cells within the colony. The latter is achieved by the entire animal rearranging itself, with the cells over each other and pulling along the spinules, so it resembles more like a blob flowing across the surface. The former is utilised by small spherical colonies, utilising filamentous pode that extend from the sponge body wall with an adhesive knob on the end. Once this attaches to the surface, the colony is pulled to the new location by the podia that contracts .
All sponges efficiently draw in water and select or reject food particles from the water. The process is an active one, with the current caused by the feating flagella on the collar cells. Water enters through inhalent pores (ostia) and filtered by a series of sieves of diminishing mesh size. Water enters the sponge itself through small pores (prosophyles) located around the base of the collar cells and passes through the small spaces betwen the microvilli on the collar cells. Here the food particles are ingested by the amoeboid cells and water products are returned to the exterior via the water vascular system and exhalant pores (oscula) in an excurrent stream of water. 
The Porifera phylum is divided into four classes:
- Calcarea - Calcerous Sponges, with mineral skeleton exclusively calcite (calcium carbonate), with both discrete spicules and fused crystalline calcite skeletons.
- Demospongiae - Siliceous Sponges or True Sponges, with mineral skeleton composed of silica spicules and/or spongin fibres. 90% of sponge species belong to this class.
- Hexactinellida - Glass Sponges, with mineral skeleton exclusively silicate, with spicules include triaxon with six points and some fuse into long siliceous fibers.
- Sclerospongiae -
- Sponges, Phylum Porifera by Rob Toonen - Advanced Aquarist's Online Magazine
- Sponges: A Bunch of Holes Held Together by Some Cells by Ronald Shimek - Reefkeeping Magazine
- Identification and Husbandry of Aquarium Sponges by Ronald Shimek - Reefkeeping Magazine
- Sponges by Rob Toonen - Reefs.org
- Do sponges have circadian clocks? (plus bit about that they do move)
- (Mather Bennett 1994): Mather, P., Bennett, I., A coral reef handbook, Surrey Beatty and Sons: Chipping Norton, 1994.
- (http://dx.doi.org/10.1007/BF00310322): Fishelson, L., Observations on the moving colonies of the genus Tethya (Demospongia, Porifera), Zoomorphology, 98(1), (1981), 89-99.
- (Ruppert Barnes 1994): Ruppert, E.E., Barnes, R.D., Invertebrate Zoology, Saunders College Publishing: Sydney, 1994.