Body Cavities
The evolution of body cavities within the kingdom Animalia has a very interesting history. In fact, the increasing complexity of animal form and function during the evolution of the group can be directly linked to the evolution of ever-more-sophisticated body cavities.
The most primitive animal phyla possess only a single body cavity, which typically has either digestive or circulatory functions, or both. There is nosecondary body cavity, or coelom, and consequently these phyla are referred to as the acoelomates.
Most animal phyla, however, have evolved a second body cavity of one form or another. The pseudocoelomates, which include a number of worm-like phyla, are characterized by a secondary body cavity known as the pseudocoelom. The pseudocoelom has some but not all of the characteristics of true coeloms. Finally, several animal phyla, including those that possess the most complex body plans in the kingdom, are characterized by a body cavity known as a true coelom. These phyla are known as the eucoelomates.
The Acoelomate Phyla
The most primitive animal phylum is that of the sponges (phylum Porifera). Sponges have a single body cavity known as the spongocoel. The spongocoel is critical to the food gathering strategy of sponges.
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