PHYLUM CNIDARIA
The Phylum Cnidaria includes such diverse forms as jellyfish, hydra, sea
anemones, and corals. Cnidarians are radially or
biradially symmetric, a general type of symmetry believed primitive for
eumetazoans. They have achieved the tissue level of
organization, in which some similar cells are associated into groups or
aggregations called tissues, but true organs do not occur.
Cnidarian bodies have two or sometimes three layers. A gastrovascular cavity
(coelenteron) has a single exterior opening that
serves as both mouth and anus. Often tentacles surround the opening. Some cells
are organized into two simple nerve nets, one
epidermal and the other gastrodermal, that help coordinate muscular and sensory
functions.
Cnidarians have two basic body forms, medusa and polyp. Medusae, such as adult
jellyfish, are free-swimming or floating.
They usually have umbrella-shaped bodies and tetramerous (four-part) symmetry.
The mouth is usually on the concave side,
and the tentacles originate on the rim of the umbrella.
Polyps, in contrast, are usually sessile. They have tubular bodies; one end is
attached to the substrate, and a mouth (usually
surrounded by tentacles) is found at the other end. Polyps may occur alone or in
groups of individuals; in the latter case,
different individuals sometimes specialize for different functions, such as
reproduction, feeding or defense.
Reproduction in polyps is by asexual budding (polyps) or sexual formation of
gametes (medusae, some polyps). Cnidarian
individuals may be monoecious or dioecious. The result of sexual reproduction is
a planula larva, which is ciliated and
free-swimming.
If collar cells and spicules are defining characteristics of the Phylum
Porifera, then nematocysts define cnidarians. These tiny
organelles, likened by Hickman to cocked guns, are both highly efficient devices
for capturing prey and extremely effective
deterrents to predators. Each contains a coiled, tubular thread, which may bear
barbs and which is often poisoned. A
nematocyst discharges when a prey species or predator comes into contact with
it, driving its threads with barb and poison into
the flesh of the victim by means of a rapid increase in hydrostatic pressure.
Hundreds or thousands of nematocysts may line the
tentacles or surface of the cnidarian. They are capable even of penetrating
human skin, sometimes producing a painful wound or
in extreme cases, death.
We recognize four Classes of Cnidaria:
These organisms may exist as either polyps or medusae. Many species pass through
both forms in their life cycles; in others,
one form or the other is suppressed. The majority of species are marine and
colonial, but many species are solitary and some
live in fresh water.
Hydrozoans include such diverse forms as hydra, which are a group of species of
solitary, fresh-water forms that live as polyps
throughout their lives. Reproduction by hydras can be the typical form for
polyps, asexual by budding, or by sexual means in
which temporary gonads appear on the polyp itself.
Hydrozoans also include colonial species, called hydroids. Hydroids are actually
colonies of polyps growing on a common
stalk. Colonies are formed by the asexual budding of members; these buds, unlike
those of hydras, remain attached to the
parent. Some polyps in a typical hydroid specialize in reproduction; these
produce medusae by budding. In this typical case, the
medusae, which are usually free-swimming, leave the colony, mature, and produce
gametes for sexual reproduction. The
gametes are shed, fertilization takes place, and the zygote develops into a
free-swimming, ciliated planula larva. After a time the
planula settles to the bottom and develops into a polyp, which by budding
eventually forms a new colony. Not all hydroids
follow this pattern; in some, the medusa phase is compressed or reduced and
gametes are shed directly from specialized
polyps.
LIFE
CYCLE OF A HYDRA
embryo —> planula —> polyp —> medusa —> gametes —> embryo
Some hydrozoans, such as the Portuguese man-of-war, form floating colonies that
include both specialized medusae and
polyps. Others, called hydrocorals, have calcareous skeletons that resemble
those of true corals.
Hydrozoan medusae all have a distinctive structure called a velum. This is a
shelf or rim that projects inward around the margin
of the bell, partially closing the opening. Hydrozoan medusae swim by
alternately constricting and relaxing muscles in the bell.
This causes water to shoot out the constricted opening of the bell, moving the
animal by a sort of "jet propulsion."
The Class Scyphozoa includes most of the larger jellyfish. The medusae of
members of this class lack a velum. In many species,
the rim of the bell contains sense organs, including statocysts that sense
balance and orientation and photoreceptors that are
sensitive to light. Scyphozoans are often amply covered with nematocysts, so
that a swimmer's encounter with one can be an
unpleasant experience. They feed in a variety of ways, but often involving prey
capture by nematocysts on the arms and
transport of food items to the gastrovascular cavity by means of cilia. As in
other cnidarians, digestion is intracellular.
Scyphozoans are frequently strikingly tetramerous (four-parted). Four gastric
pouches connect with the gastrovascular cavity,
and the opening to that cavity may have four lobes. The peripheral sense organs
are usually placed in pits or notches; these are
often present in multiples of four.
Scyphozoans are dioecious. Fertilization and early development usually take
place in the gastrovascular cavity or on the lobes
near the gastrovascular opening. A ciliated planula larva forms; in most
scyphozoans it settles and forms a hydra-like polyp.
This life stage, called a scyphistoma, reproduces asexually by budding. At first
the product of budding is other scyphistomas,
but later they produce tiny medusae that break free and grow to form mature,
sexual scyphozoan medusae.
LIFE
CYCLE OF A JELLYFISH
embryo —>
planula —> scyphistoma —> medusa —> gametes —> embryo
Cubozoans are also known as box jellyfish, because in transverse section the
bells appear to be square. Tentacles are located
at the corners of the square umbrella margin, and the base of each tentacle is
distinctively flattened. The edge of the umbrella
turns inward to form a rim called a velarium, much like the velum of
hydromedusae.
Cubozoans are considered by some to be a subclass of Scyphozoa. Effective
predators and strong swimmers, they feed mostly
on fish. Their nematocysts are especially potent, sometimes resulting in the
death of unfortunate human swimmers who
encounter them.
The Class Anthozoa includes a variety of animals that have polyps with a
flower-like appearance. In these forms, the
gastrovascular cavity is large. It is divided by walls or septa, which arise as
folds from the body wall. These folds, along with
the mouth and pharynx, are usually arranged in a biradially symmetric pattern.
Anthozoans include sea anemones, a variety of corals, sea fans, and sea pens.
Sea anemones are carnivorous polyps that are
quite large, ranging up to 200mm in length. They tend to be brightly colored.
Most species live in warm water. They feed on
fishes, which are caught by means of the numerous nematocysts in their tentacles. These animals are known for their symbionts.
These include species of fish that actually live among the tentacles of large
anemones, somehow avoiding lethal contact with the
nematocysts. Other anemones have unicellular algae living within their tissues,
from which they probably derive some nutrition.
Yet others have a symbiotic relationship with hermit crabs, which gather up the anemones and place them on the snail shells that
the crabs occupy. The anemones benefit from food particles dropped by the crab, and the crab gains protection from predators due
to the presence of the nematocyst-laden anemones.
The Class Anthozoa also includes many kinds of corals, including many
reef-building species. Reefs are formed by the
calcareous skeletons of many generations of coral polyps. The polyps inhabit
only the surface of the reefs. These reefs are
among the most productive environments of the world, housing thousands of
species of fish and invertebrates, not to mention
plants and protists. Like some anemones, corals are inhabited by symbiotic algae
called zooxanthellae. These photosynthetic
algae are essential for the coral, and corals generally do not live at depths to
which light does not penetrate.