Sea sponges belong to the phylum Porifera. They are a broad group of simple multicellular organisms and are thought to be basal to all other animal group. In other words, their lineage predates all other animal life and they likely have many traits in common with the first animals. Fossil evidence and DNA analysis indicates that sponges arose at least 800 million years ago.
Most animals have a distinct symmetry. Humans and many other animals, including insects, worms, and other vertebrates, have bilateral symmetry. This means the left half of the animal is a mirror image of the right half. Some animals such as jellyfish and starfish display radial symmetry: you can bisect the animal along two or more lines to get equal halves. Sea sponges are unique among animals in that they have an unspecific bauplan (body plan) with no overall symmetry. Individual subunits of the sponge may still display symmetry at both microscopic and macroscopic levels.
The vast majority of sponges are technically asymmetrical. This means the sponge has no apparent bauplan. The cells of most sponges are only loosely interconnected and can be separated by physical disruption. If allowed to resettle, the separated cells will regroup in a form similar to their original structure. This indicates that there is nevertheless some form of genetic or other molecular signal that determines the overall structure of the sponge. Many sponges have amorphous bodies but display some form of symmetry in their individual subunits, such as the spicules seen in some species.
There are no sponges that display "true" radial symmetry as adults, though there is some question as to whether the common ancestor of sponges displayed radial symmetry. Some species of one of the more primitive classes of sponge, Calcarea, display radial symmetry during development. In these species, the developing sponge forms a circular plate shape with a distinct cross-shaped tetraradial (four-way) symmetry.
Partial Radial Symmetry
Several groups of poriferans, particularly sponges in class Hexactinellida, display partial radial symmetry. Hexactinellids are a class of deep-water sponges containing a silica-based skeleton to keep the animal's body rigid. They are generally flower-shaped and have five or six radial spicules in their skeleton.
Partial symmetry means that there are often numerous asymmetrical offshoots from the main spicule (central structure) and this symmetry isn't crucial for everyday function. In addition to the hexactinellids, several other groups of sponges such as Demospongiae and the now-extinct class Archaeocyatha display this type of symmetry on some level.
Some species of sponge display false symmetry. This is frequently seen in species of class Demospongiae. These animals display outward symmetry and appear to be bell- or flower-shaped. Despite this appearance, careful analysis of the sponge's interior skeleton and aquiferous system (series of internal tubes) shows that there is no internal symmetry. Thus, this is called false symmetry.
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