Clathria basilana

   Porifera are commonly referred to as sponges.  An early evolutionary branching event in the history of animals may have separated the sponges from other metazoans, and from there the sponges seem to have traveled their own separate evolutionary route.  Therefore, they are frequently placed in their own subkingdom, Parazoa, meaning "beside the animals".  Until the nineteenth century, sponges were considered a plant-animal, because during their adult life they are all sessile (attached to something else, like a rock; not free to move about).

    Fossil sponges are among the oldest known animal fossils.  The living sponge species are classified in the phylum Porifera, which is composed of three distinct groups, the Hexactinellida (glass sponges), the Demospongia, and the Calcarea (calcareous sponges). There is a fourth, small group, known as Sclerospongiae, which has the characteristics of all three of the other groups combined, but it is generally not mentioned as a major group.

    Sponges can be found on ocean floors in most places around the world.  Most live along the coast in shallow water, but some, such as the fragile glass sponge, are found very deep in the ocean, where water currents are very slow and do not disturb them.  A few sponges can even be found in fresh water.

    There are approximately 5,000 living species of sponges.  A sponge is essentially a water-filtering system.  The sponges don't move and all species must feed on food particles suspended in the water. In all cases, poriferans have a canal system, through which they pump water. Water enters through pores called ostia, flows through canals to a spacious chamber called a spongocoel, and finally exits through large openings called oscula. Sponges have no specialized reproductive, digestive, respiratory, sensory, or excretory organs found in this group.  Most often, they are asymmetrical, amorphous creatures, although some spectacular basket and vase-shaped species do occur. Sponges consist of 3 layers: (1) an outer layer of flattened contractile epithelial cells (pinacocytes); (2) an inner, non-living gelatinous layer containing amoebocytes, which perform several functions in reproduction, secrete skeletal materials, and, most importantly, carry food particles from the choanocytes to the epithelial and other non-feeding cells; and (3) an inner layer of flagellated collar cells (choanocytes) that circulate seawater within and through the sponge to bring in food and reproductive products and help discharge waste products.
 
 

    The living sponges of today are grouped into four different classes, according to their skeletal structure, which serves for protection, stiffening, and support:

Class Calcarea--the skeleton consists of individual spicules of calcium carbonate

Class Hexactinellida (the glass sponges)--members have spicules of silica fused in a continuous and often very beautiful latticework

Class Demospongiae--the largest class, which has unfused silica spicules, OR a tough, keratin-like protein called spongin, OR a combination of the two
 

Class Sclerospongiae--the smallest class, which have skeletons that contain all three kinds of material: calcium carbonate, silica, and spongin
 
 
 
 

    The reproduction of sponges exhibits many characteristics of sessile or slow-moving animals. Asexual reproductionis quite common, and occurs in one of two ways:
1.) fragments that break off from the parent animal may become new sponges
      OR
2.) gemmules: collections of amoebocytes within a hard, protective outer layer

Sexual reproduction in sponges is highly specialized.  The simplest and most primitive form of fertilization is external, with the sperm and egg cells shed into the water.  However, in most sponges, fertilization is internal.  The sperm cells are carried by the water currents out of the osculum of one sponge and into the interior cavity of another sponge.  There they are captured and transferred to ready eggs.  Most sponges even provide a certain amount of maternal care, retaining the toung during the early stages of development.  The embryonic  (not full grown, still developing) sponge develops into a free-swimming larva that locates an appropriate site, settles and attaches, and develops into an adult sponge.
     Most kinds of sponges are hermaphrodites, meaning the same individual has both male and female reproductive structures and produces both sperm and egg cells.  This is a great advantage for animals with little or no motilitty, as a hermaphrodite can mate with any partner, with no limitations concerning gender.
 

Sponges are very interesting and unique, with many different and beautiful species.  For a much more detailed account, with tons of interesting information, check out these great sites: