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Go to the key ---INDEX---
Major Phyla
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General fossil discussion
Geologic time
Phylum PORIFERA (the sponges)
Sponges are intermediate between organisms with independent non-cooperating
cells, and organisms with tissues where the cells cooperate or act as a
unit. When separated, sponge cells behave like protozoans.
Unlike protozoan cells, however, they will die after a period of time if
prevented from rejoining to form a new sponge. In more complex organisms
with tissues such as coelenterates the cells die immediately when separated.
The shape of sponges is variable. Many are nondescript while others
are globular or cylindrical. They all have skeleta composed
of organic fibers and/or calcareous (calcite) or siliceous (quartz) rods
called spicules. It is these spicules which are most often preserved.
These star-shaped spicules are particularly conspicuous in the genus Astreospongia.
Phylum ARCHAEOCYATHA
Archaeocyathids resemble sponges but are conical and have double walled
skeleta. They are a short-lived group that appeared in the Early Cambrian,
commonly formed reefs, and disappeared in the Middle Cambrian.