Filter feeding is accomplished using specialized feeding cells called choanocytes (collared cells) that have a central beating flagella (a little whip-like tail) ringed by small filaments. They do not have protective shells to live inside, but rather depend on tiny little skeletal elements called spicules to give their bodies rigidity. They come in a remarkable variety of sizes, colors and shapes, but all are immobile and live on the sea floor. Sponges are so basic in their design that there is some argument over whether their different cells can even be described as being organized in to discrete tissues. Sponges are ancestrally primitive animals (first considered plants, Aristotle identified their animal nature) that while simple in their construction, have a number of tricks up their sleeves (well actually, up their ostia, the little pores that lead to channels that radiate through their bodies bringing food and fresh sea water for respiration). One of the fascinating groups of marine invertebrates that we study here in Antarctica are the marine sponges (yes, there is also a small group of freshwater sponges).
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