Phylum Porifera

Common Name: Sponges

Etymology: "Pore-bearers", from the Latin porus for pore and ferre to bear, hence an animal with with pores. Obviously, this refers to the fact that their outer cell layer is perforated by many small pores through which water is drawn.

Representative examples:

Click images to enlarge.
Leucetta, a shallow-water calcareous sponge	Euryspongia, a demosponge	Agelas, a demosponge	The freshwater demosponge Spongilla	Colonial sponges. Water flows into the central body cavity of each of the sponges through the numerous pores in the body walls, and is discharged through the oscula (the larger openings).

Symmetry: None

Tissue Layers: None

Coelom: None

Digestive Tract: None

Special Features:

Choanocytes = "collar cells", line the interior body walls of sponges and contain a central flagellum surrounded by a collar of mucus-coated microvilli. The flagellum beats regularly, creating a water flow across the microvilli which can then filter nutrients and other food from the water. The similarities between choanocytes and flagellated protists suggest an evolutionary relationship between the two groups.

. click to enlarge

Additional Information:

Although they are multicellular animals, the sponges are thought to have diverged from other metazoans very early. They are classified in the Parazoa, as opposed to the Eumetazoa, because they lack true tissues, a fundamental developmental difference from all other multicellular animals. Most cells in the sponge body retain totipotency and on many levels the sponges function like organisms with unicellular grade complexity despite their relatively large size and multicellularity.

As a group, poriferans lack symmetry (although some tube sponges and other forms display loosely radial symmetry) and have a body composed of two layers of cells, each only 1-cell thick. Choanocytes, or collar-cells, on the inside layer are flagellated and create a current of water through the sponge from which organic matter is filtered for food. Food trapped in mucus coated microvilli on the collar cells is ingested by phagocytosis and digestion is intracellular (unlike most metazoans). Lacking organs, excretion and gas exchange occur via simple diffusion. Sponges also lack nervous systems and discrete sensory organs.

Nearly all sponges are marine and bottom-living and are essentially sessile.

Links:

• Introduction to the Porifera (Univ. of California Museum of Paleontology)
• Porifera: Life History and Ecology (Univ. of California Museum of Paleontology)
• Phylum Porifera (Palaeos)
• Earthlife Invert Diversity, Phylum Porifera