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In many ways, animals from the Phylum
Cnidaria
seem very similar to sponges: these animals
also have an
outside (ectoderm or epidermis) and inside (endoderm
or gastrodermis) cellular layer with a jellyish layer with roaming replacement amebocytes in between, and many Cnidarians are sessile,
with swimming sperm and sometimes swimming larvae. However,
thats about as far as the resemblance goes.
Cnidarians are almost exclusively predators,
catching and eating other animals. The characteristic
feature of the groups, their cnida, are
devices
through which they capture prey: they might deliver a
paralyzing sting, a capturing harpoon and cable, or a tangling
net. The poisonous sting, called
nematocysts,
is the most common type of cnida. The old name for the
phylum was Coelenterata, a name based upon their internal
space, but there was nothing especially unique about that
structure, while cnidae, the basis for the new name, are not found in any other group.
Although Cnidarians are often not considered
complex enough to have tissues, they do have strands of cells that
act as muscles and a nervous system, although there are no
processors or "brains." They almost always have a
ring of tentacles around the mouth,
which opens to allow food to be pushed into an inner space called
the
gastrovascular cavity. The
"gastro" part of the name is from the fact that
digestion occurs there ("gastro" = "stomach"),
and the "vascular" part is from the extension of the
hollows to all parts of the body for distribution of nutrients and
oxygen, like a vascular (tube distribution) system. The body
around the gastrovascular cavity can be varied in shape, but it always has
a pattern called
radial symmetry: a circular pattern
where the features in any "pizza slice" will more-or-less
duplicate those in another slice the same size. Some animals,
including several types of Cnidarians, have radial symmetry that follows a
specific numeric pattern;
sea anemones, for instance,
are octoradially symmetrical, with any one-eighth slice equivalent
to any other one-eighth slice.
Two main
body plans can be found in the
Cnidarians:
polyps,
with the tentacles and mouth on top, and usually a column-shaped
sessile body, and
medusae,
with the tentacles and mouth underneath, and usually a bell-shaped
body that can pulse and swim - a jellyfish. There are some
groups with just one or the other, and some with both, and no one
knows for sure which shape came first - the oldest fossils include
both.
In the Cnidarian groups that have both forms, a
type of alternation
of generations is followed that allows a sessile
animal to spread great distances. The polyp forms are the asexual
stage, able to make lots and lots of polyps and spread
locally, and to asexually produce the
genetically-identical-but-physically-different medusae. The
medusae are the sexual stage, able to swim away from the
parent (and drift long distances as plankton),
find other medusae to mate with, and drop larvae where they might
be able to colonize an entirely new area, such as near islands
newly-risen from the sea floor.
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