Porifera, a phylum within the Kingdom Animalia, consists of all species of sponges, about 5,000 total. Sponges are among the

simplest animals on the planet, and it is believed that they evolved from the first multi-cellular organisms which eventually evolved

into animals. The phylum Porifera represents the only phylum of the Parazoa branch of Kingdom Animalia--those without true

tissues, that diverged from the lineage shortly after the evolution of multicellularism from colonial protists.  Sponges are placed

 in the subkingdom Parazoa because of their lack of true tissues.  Early scientists originally thought sponges to be plants, or at least

 related to plants because of the similarities in their looks.  (They look like plants.)  Closer examination, however, revealed that the

sponges were heterotrophic, and had to be placed in a different category as the plants.


The majority of sponges are considered to be radially symmetrical. This means that they are divided into two congruent halves by

any longitudinal cut through the center of the organism. Actually, no sponges are truly radially symmetrical, because they have no

true tissues.


Most sponges also have three "layers" of cells. The outer layer is made up of flattened cells with numerous pores throughout. The

layer of cells contains mobile cells as well as the skeletal cells which give the sponge its shape. The skeletal cells are either

spongin, a flexible protein, or simply mineral-containing particles. The third, innermost layer is made completely of cells called

choanocytes. These cells have a ring surrounding the base of a flagella, which is used to propel water through the pores of the

sponge into a central body cavity, and then out of the sponge via a central opening. The second function of a choanocyte is to trap

food, mainly bacteria, on its collar, and absorb it by phagocytosis.


The mobile cells in the center layer are called amoebocytes. These cells are able to move by use of pseudopodia. They are

constantly moving, and their main function is to pick up food vacuoles from the choanocytes, digest the food inside them, and carry

the nutrients to the cells that need them. However, they also function to carry oxygen to other cells, dispose of waste products,

maintain the structure of the sponge, and even change into another cell type if they have to.


Sponges have a number of similarities to protists, even though they are definitely animals. Sponges have no digestive tract of any

kind (instead, they digest food within cells), they lack the gastrula stage, the three cell layers are not homologous to the body layers

of most animals, and they do not have any nerve cells or muscle cells. The probable reason for this similarity to Kingdom Protista is

because of their common ancestor. It is believed that the ancient ancestor of sponges is a choanoflagellate, a colonial protist.

Choanoflagellates live at the bottom of shallow water, and consisted of a small cluster of flagellated cells incredibly similar to a

choanocyte. This similarity means that sponges probably evolved from this type of colonial protist, which would explain the similarity

to protists.



Phylum Porifera includes the following classes:


     Class Calcarea (Containing rigid calcium carbonate)

     Class Demospongia (The vast majority of sponges)

     Class Hexactinellida (Many extinct, contain silica spines)  



Calcarea is the only class within Porifera which is made of spicules composed of calcium carbonate, a mineralized substance.

Unlike Demosponges, Calcarean spicules lack hollow canals, making calcareans stronger, in general, than demosponges.

Calcareans are found predominantly in shallow, tropical water near a shore, but some species can live in marine environments at a

depth of 4000 feet.



Demospongia is the largest and most diverse of the classes of sponges, comprising approximately 90 percent of all sponges. The

spicules of a demosponge can be made of either spongin, an organic substance, or silica, a mineralized substance, and in some

cases, both. Demosponges can grow to be very large, and are found in virtually any tropical or subtropical water, as well as in other

marine environments.



Hexactinellida is a class of sponges with spicules, which make up sponges, of silica. Silica is a mineralized substance, hence the

common name for hexactinellids is glass sponges. These spicules join with each other at right angles, making the sponge appear

artificial and thus they do not have the ability to contract. Hexactinellida is considered an early branch within sponges, and most

species are extinct, but some do still exist.





¨      Why are sponges sometimes referred to as the "parazoa?"


Sponges have traditionally been considered to have evolved from a different ancestor than the rest of the animal

kingdom and are therefore sometimes said to have evolved "in parallel."  Generally, the term "Parazoa" is not seen

much in modern schemes.  For now, sponges retain their position in the kingdom animalia although some evidence

suggests that the porifera, cnidaria, and certain protists called choanoflagellates are closely related and form a

clade of their own, separate from the lineage that led to other animals.



¨      Why are there almost no large bath sponges left in the ocean?


Collecting bath sponges is highly profitable because they are so easily caught ("harvested" would probably be a

better term since a sponge doesn’t put up much struggle).  Unfortunately, sponge growth rates are typically low.

The largest sponges were considered most desirable so were collected first during the last century.  Very soon, all the

large bath sponges were collected and today, very few survive.



¨      Why do sponges have few predators?


The living material of a sponge is thinly distributed through a scaffold of indigestible skeletal material.  Eating a

sponge would be a bit like eating a mouthful of glass toothpicks, or a bundle of fishing line.