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.
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