Animal World

Sponges, Coelenterates, and Worms

What is the most primitive group of animals?

Sponges (phylum Porifera, from the Latin terms porus, meaning “pore,” and fera, meaning “bearing”) represent the most primitive animals. These organisms are aggregates of specialized cells without true tissues or organs, with little differentiation and integration, and with no body symmetry. A sponge’s body is perforated by holes that lead to an inner water chamber. Sponges pump water through those pores and expel it through a large opening at the top of the chamber. While water is passing through the body, nutrients are engulfed, oxygen is absorbed, and waste is eliminated. Sponges are distinctive in possessing choanocytes, special flagellated cells whose beating drives water through the body cavity and that characterize them as suspension feeders (also known as filter feeders).


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