The Phylum Cnidaria is made up
of members who's body plan is a bag within a bag. The
outside skin, or exoderm, has nematocysts, cells that can
sting and protect the animal: the inside. or ectoderm,
holds the food that the animal has captured and digests this
food. It expels what it
cannot digest through the opening
in the top of its body.
Between the ectoderm and the endoderm there is a space
filled with a soft jello-like material called the mesoglea.
This material is not inside cells, it is a sort of filler
that can absorb oxygen and nutrients and redistribute these
by diffusion. It makes the walls of the anenome
thicker, which strengthens them. The mesoglea also has
collagen in it, which strengthens the walls and helps the
animal to maintain its shape.
introduced something new: tissues, which are groups
of cells that have a specific, differentiated function.
The cnidaria were the first animals to have simple muscles
and nerves to allow them to contract the sides of their
bodies. They need to do this in order to expel
non-food items from their bodies.
This was a great
advance in the evolution of animals!
Their simple body plans
Starting from this
basic plan, many different animals evolved. All the
animals that descended from the distant common ancestor
continued to have the nematocysts, the stinging cells.
Their physical forms evolved to find different ways to live
in the ocean. We will look at three of them: sea
anemones, corals, and jellyfish.
Sea anemonies clearly have
the bag within a bag shape. Although they look like
flowers when their tentacles are open, they are really quite
fierce predators, spearing their prey with the stinging
cells (nematocysts) in their tentacles. The ones that
feed on plankton have delicate, feathery tentacles. those
that seek larger prey have stronger tentacles that can pull
small fishes into the anemone's digestive cavity. The
anemone can close off its top and digest its meal, then it
spits out whatever is left over.
Sea anemones are found in all ocean
environments, even in the great depths of the abyssal plain.
They are more plentiful near the shores.
Anemones reproduce sexually. The
eggs are fertilized in the gastric cavity and later released
as free-swimming larvae. The young anenomes later
attach themselves to rocks or other solid objects.
Anemones can also reproduce by budding. A little
anemone can just bud off from the side of an adult.
Anemones absorb oxygen from the water
Sea anemones can move by creeping on their
basal disks, but this is such a slow process that it is not
really visible if you sit down to watch it.
There are over 1000 species of anemones in
earth's oceans today. That's a success story!