SPONGES

In many ways, lacking true tissues and organs, sponges are unlike any other multicellular animal. Yet, they share certain genes with cnidrians (corals and anemones). Classified in the phylum Porifera, they are filter feeding animals that grow in upright or encrusting forms. They are generally sessile as adults, although a few are slightly mobile. Sponges can reproduce asexually, by budding, and sexually. Some are even capable of regeneration. There are about 5000 species world wide, mostly marine. Its skeleton can be made up of hard mineralized spicules of calcium carbonate, silicone dioxide or by flexible spongin fibers. Countless microscopic spores on the outer edge of the surface carry water via tubes (incurrent canals or spores) to tiny chambers lined with flagellum-whirling choanocytes or "collar cells". These cells propel water currents from which fine particles and bacteria are filtered out and their nutrients absorbed. Water then flows out to one or a few large excurrent openings, osculums, and back into the ocean. A baseball-sized sponge may pump a liter of water per hour, extracting over 95 percent of the bacteria from the flow. The cell structure contains only a few types of cells, many which have the ability to transform into other cells. Few animals eat sponges.

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