"Sponges
are the simplest multicellular animals. Because the cell is the elementary unit of life, the evolution of organisms
larger than unicellular protozoa arose as an aggregate of such building units. [...] There are many advantages
to multicellularity as opposed to simply increasing the mass of a single cell. Since it is at cell surfaces that
exchange takes place, dividing a mass into smaller units greatly increases the surface area available for metabolic
activities. [...] Thus multicellularity is a highly adaptive path toward increasing body size."
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Sponges
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 are 5000 species of sponges known across the world.
Sponges are primarily marine, but around 150 species live in fresh water.
An early
branching event in the history of animals separated the sponges from other metazoans. Fossil sponges are among
the oldest known animal fossils, dating from the Late Precambrian. However, they differ from all higher multicellular
animals in that their cells are not arranged to form permanent tissues or organs.
Sponges
are sessile organisms. Water movement is driven by the beating of their flagellae, which are located on specialized
cells called choanocytes (collar cells). Sponges reproduce by sexually and asexually.
The website below contains more information and photos:
http://www.ucmp.berkeley.edu/porifera/porifera.html
http://www.ucmp.berkeley.edu/porifera/porifera.html
http://animaldiversity.ummz.umich.edu/porifera.html
http://www.york.biosis.org/zrdocs/zoolinfo/grp_spon.htm
http://cbs.umn.edu/class/biol/2012/porifera.htm
http://www-biol.paisley.ac.uk/courses/tatner/biomedia/units/pori1.htm
For more information see