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JELLY ANIMALS: CNIDARIANS AND CTENOPHORES
GENERAL OVERVIEW
Cnidarians and Ctenophores are invertebrates, animals
without backbones. Like all animals, they are composed of
many cells, must eat to gain energy and can move from
place to place during some part of their lives. Both these
types of animals have radial symmetry: their body parts
radiate out from the center and each slice is a duplicate of
the other. This allows them to sense prey or enemies from
any direction but with no front,also makes it difficult to
coordinate their bodies to move towards a special spot.
Animals that are radially symmetrical are generally slow-moving and have very slowmoving or anchored prey.
Both Cnidarians and Ctenophores share other characteristics: They are made up of only 2 tissue layers: an
outer protective covering containing nerve cells in a net-like arrangement and an inner layer containing digestive
cells and reproductive organs, their only organ system. Cnidarians and Ctenophores may be up to 95% water!
The space between the two tissue layers is filled with a jelly-like substance for support and movement. In the
middle of both kinds of animals is a sac-like gut with only one opening: food goes in and waste goes out the same
opening! They do not have Cephalization like most animals, which means they lack a definite head end, with
feeding and sensory structures.
I.
Cnidarians (Phylum Cnidaria)
This phylum includes stinging hydroids like the Portuguese-Man-of-War,
jellyfishes, and corals and sea anemones. Cnidarians may spend their lives as
one of two body forms:
A. The medusa resembles an umbrella and floats like a tentacle-fringed bell
in the water, surrounding the central mouth. Medusa jellyfish are planktonic (which
means they cannot swim against a current) and move by pulsating the jelly in their bell.
B. The polyp is tubelike and is usually attached to the ocean bottom; it may be solitary or part
of a colony. Polyp forms move by pushing the water in their guts to slowly ooze across a
surface.
FEEDING:
Stinging cells are filled with toxins that can stun or kill prey. The
harpoon-like nematocysts lie coiled inside out within the stinging
cells and shoot outward on contact or chemical stimulation. Once
captured, tentacles bring prey into the hollow gut for digestion. Waste
is excreted out same opening!
IMPORTANCE:
Cnidarians major predators on zooplankton and larger inverts and fish; often eaten as prey themselves,
especially by sea turtles. Many Cnidarian are toxic to humans: Thousands of humans are stung each year by
PORTUGUESE MAN OF WAR, SEA WASPS (the most deadly), and FIRE CORAL. Cnidarians are used in
many biological & medical studies and colorful HYDROCORALS are often collected for saltwater aquariums.
Coral reefs are largest structures built by living organisms on the planet and support an incredible species
diversity. Coral colonies can provide borrowing spaces and protection for a wide range of animals and plants.
They protect island margins from high waves and surge and are prime eco-tourism spots and the wealth of sea life
in and around reefs supports commercial fishing industries
II. Comb Jellies (Phylum Ctenophora)
This phylum includes planktonic jelly animals that are generally small and almost
transparent. Many spend their lives in the darkness of the deep ocean and can
biolumenesce to make their own living light for prey capture or avoidance.
Ctenophores move by beating 8 rows of tiny comb-like cilia along their bodies,
which shine like rainbows in the darkness of the ocean.
FEEDING:
Instead of having many tentacles and stinging cells for catch their food, Ctenophores have 2 long
tentacles, often coated with a paralyzing toxin. When its prey comes to close, it may get tangled in the
tentacles and be slowly reeled into the Ctenophores mouth. Some Ctenophores have teeth-like spikes
lining the opening of the mouth and open their mouths wider than their bodies to gobble down prey bigger
than themselves.
IMPORTANCE: Ctenophores are major planktonic predators and are often eaten as prey themselves.
Ctenophores are not as dangerous to humans as Cnidarians: the toxin coating the long tentacles needs a break
in a person's skin to seep in.