Exception to the Mostly Asymmetrical Nature of Porifera

Porifera, commonly known as sponges, are mostly asymmetrical animals. Asymmetry refers to the absence of symmetry or a lack of definite shape or organization. However, there is one exception to this general rule found within the phylum Porifera.

Symmetry in Porifera
Most sponges exhibit a lack of symmetry, which means they do not possess any particular body shape or organization. This is due to their cellular structure and the absence of specialized tissues or organs. The body of a sponge is composed of a mesh-like structure called a mesohyl, which is comprised of cells called choanocytes and amoebocytes, along with a gel-like substance called mesoglea. These cells are loosely organized and have a random arrangement, resulting in the asymmetrical nature of sponges.

Exception: Radial Symmetry in Some Poriferans
Although most sponges are asymmetrical, there is one group of poriferans that exhibit radial symmetry. The class Demospongiae, which comprises the majority of sponge species, contains a small number of species that possess radial symmetry. Radial symmetry is a type of symmetry in which body parts are arranged around a central axis, similar to the spokes of a wheel. This arrangement allows for equal division of the body into multiple similar parts.

This exception to the asymmetrical nature of sponges can be observed in certain species within the class Demospongiae, such as the genus Euplectella (also known as the Venus' flower basket). These sponges have a cylindrical shape with a central axis and radial symmetry. The body of Euplectella is composed of a network of silica spicules, which provide support and structure. The radial symmetry in these sponges allows them to efficiently filter food particles from the water currents that pass through their bodies.

Conclusion
While the majority of poriferans are characterized by their asymmetrical nature, there is an exception within the class Demospongiae, which includes a small number of sponge species that exhibit radial symmetry. This exceptional feature allows for a more organized arrangement of body parts, which can be advantageous for filter feeding and survival in their respective habitats.

Radial symmetry in Sycon and Leucosolenia