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3. Porifera.

Porifera means “animals with the body perforated by pores”. They are also known as “sponges”. Porifera are aquatic sessile animals that live attached to the sand or the rocks in oceans and seas all over the word. Only a few species live in freshwater. They are considered the simplest organisms in the animal kingdom. In fact it took some time to realize that they were not a weird type of algae or fungi, and it was not until 1825 when the studies of their structure under the microscope and the nature and characteristics of their cells convinced the scientists that they were animals. Sponges present a great variety of shapes and colors and most of them are asymmetrical; but many of them look like a “cup” or “chimney” with a big internal cavity called atrium and a big hole in the top called osculum. Sponges have an internal skeleton formed by microscopic needles called spicules. The chemical composition of these spicules is used as a criterion to classify these animals. Most of the times the spicules are made of calcium carbonate or silicon dioxide but sometimes they are made by a fibrous substance called spongin. The sponges with spongin can be used as a hygienic instrument for humans. Sponges are filtering animals. They have specialized cells called choanocytes with a flagellum that is used to pump water through the inhalant pores into the atrial cavity. The small particles of food enter with the water and are captured by the cells that digest them and distribute the nutrients to the rest of the cells of the body of the sponge. The waste materials are discarded to the atrium where the current of water will take them out by the osculum.