A Fearsome Tiny Weapon
|Tentacles of a Caribbean sea anemone,
Although members of the phylum Cnidaria are more highly
organized than sponges, they are still relatively simple animals.
Most are sessile; those that are unattached, such as
jellyfish, can swim only feebly. None can chase their prey.
Indeed, we might easily get the false impression that the
cnidarians were placed on earth to provide easy meals for
other animals. The truth is, however, many cnidarians are
very effective predators that are able to kill and eat prey that
are much more highly organized, swift, and intelligent. They
manage these feats because they possess tentacles that bristle
with tiny, remarkably sophisticated weapons called
As it is secreted within the cell that contains it, the
nematocyst is endowed with potential energy to power its
discharge. It is as though a factory manufactured a gun,
cocked and ready with a bullet in its chamber, as it rolls off
the assembly line. Like a cocked gun, a completed nematocyst
requires only a small stimulus to make it fire. Rather
than a bullet, a tiny thread bursts from a nematocyst.
Achieving a velocity of 2 meters/sec and an acceleration of
40,000 × gravity, it instantly penetrates its prey and injects a
paralyzing toxin. A small animal unlucky enough to brush
against one of the tentacles is suddenly speared with hundreds
or even thousands of nematocysts and quickly immobilized.
Some nematocyst threads can penetrate human skin,
resulting in sensations ranging from minor irritation to great
pain, even death, depending on the species. A fearsome,
but wondrous, tiny weapon.
Position in Animal
The two phyla Cnidaria and Ctenophora
make up the radiate animals, which are
characterized by primary radial
or biradial symmetry
, which we believe is
ancestral for eumetazoans. Radial symmetry,
in which the body parts are
arranged concentrically around the oralaboral
axis, is particularly suitable for
sessile or sedentary animals and for freefloating
animals because they approach
their environment (or it approaches
them) from all sides equally. Biradial
symmetry is basically a type of radial
symmetry in which only two planes
through the oral-aboral axis divide the
animal into mirror images because of the
presence of some part that is single or
paired. All other eumetazoans have a primary
bilateral symmetry; they are bilateral
or were derived from an ancestor that
Neither phylum has advanced generally
beyond the tissue level of organization,
although a few organs occur.
In general, ctenophores are structurally
more complex than cnidarians.
- Both phyla have developed two welldefined germ layers, ectoderm and
endoderm; a third, or mesodermal, layer,
which is derived embryologically from
the ectoderm, is present in some. The
body plan is saclike, and the body wall
is composed of two distinct layers, epidermis
and gastrodermis, derived from
the ectoderm and endoderm, respectively.
A gelatinous matrix, mesoglea,
between these layers may be structureless,
may contain a few cells and fibers,
or may be composed largely of mesodermal
connective tissue and muscle fibers.
- An internal body cavity, the gastrovascular
cavity, is lined by gastrodermis
and has a single opening,
the mouth, which also serves as an
- Extracellular digestion occurs in the
gastrovascular cavity, and intracellular
digestion takes place in the gastrodermal
cells. Extracellular digestion
allows ingestion of larger food
- Most radiates have tentacles, or
extensible projections around the oral
end, that aid in food capture.
- Radiates are the simplest animals to
possess true nerve cells (protoneurons),
but the nerves are arranged as
a nerve net, with no central nervous
- Radiates are the simplest animals to
possess sense organs, which include
well-developed statocysts (organs of
equilibrium) and ocelli (photosensitive
- Locomotion in free-moving forms is
achieved either by muscular contractions (cnidarians) or ciliary
comb plates (ctenophores). However,
both groups are still better
adapted to floating or being carried
by currents than to strong swimming.
- Polymorphism* in cnidarians has
widened their ecological possibilities.
In many species the presence of both
a polyp (sessile and attached) stage
and a medusa (free-swimming) stage
permits occupation of a benthic (bottom)
and a pelagic (open-water) habitat
by the same species. Polymorphism
also widens the possibilities of
- Some unique features are found in
these phyla, such as nematocysts(stinging organelles) in cnidarians and colloblasts (adhesive organelles) and ciliary comb plates in ctenophores.
* Note that polymorphism here refers to more
than one structural form of individual within a
species, as contrasted with the use of the word
in genetics, in which it refers to different
allelic forms of a gene in a population.