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Unit 18: Biology Part 1 Page 16 of 60
simple types of invertebrates; Cnidarians also are radially
Sponges and Cnidarians. symmetrical. This means they
have a circular body plan such that
Sponges (Phylum Porifera) any way you cut them in half the
Sponges, found in oceans all over sides will be equal.
the world are made up of colonies There are
of specialized cells—some help two basic
push water through the sponge, body forms;
some help it feed, some are polyp and
responsible for reproduction, etc. medusa. The
They feed by filtering water polyp is a
through its pores. In this sense, cup-shaped body—with the mouth
they are known as sessile filter facing upward (sea anemones, for
feeders. Sponges are attached to example). The medusa is bell-
the ground below them—they are shaped and has a downward-facing
sessile (unable to move). mouth (jellyfish, for example).
Besides their specialized cells, A Cnidarian
sponges do not have any organs, with a polyp
nerves, or even true tissues. They body plan,
are held together through the a sea
cooperation of the colony. anemone,
and a
Cnidarians (Phylum Cnidarian with a medusa body
Cnidaria) plan, a jellyfish. Notice the upward
Painful stinging cells, called facing mouth of the polyp, and the
nematocysts, make the phylum downward facing one of the
Cnidaria especially notable. jellyfish.
Jellyfish and sea anemones are just Cnidarians, unlike sponges, have
two examples of these painful true tissues. They digest their food
creatures. The nematocysts are, in in a gastrovascular cavity—a
fact, long hollow threads that are large cavity containing digestive
used to trap prey. Additionally, enzymes. The digestive cavity also
these nematocysts are triggered has circulatory functions.
externally, so even if you come Cnidarians, like sponges, can form
across a dead jellyfish don’t step colonies. Colonies of polyps form
on it!
the jellyfish the Portuguese Man-O-
War. Similarly, colonies of
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