Phylum: Cnidarians - Cnidaria (pŕhlivce)
The phylum Cnidaria contains 10,000 species characterized by adult bodies having radial symmetry.
Cnidarians are aquatic, mostly all marine. The cnidarian body has only the ectoderm and endoderm
tissue layers, making this group diploblastic. Members of this phylum all have stinging cells that eject a
barbed thread and possibly a toxin. Only cnidaria have these cnidocytes, a specialized cell that contains
a nematocyst, a fluid-filled capsule containing a long, spirally coiled hollow thread. When the trigger
of the cnidocyte is touched, the nematocyst is discharged. Some threads merely trap a prey or predator,
while others have spines that penetrate and inject paralyzing toxins. These toxins make some jellyfish
(and a related group the box jellies) among the most poisonous of animals.
Cnidarians have two body forms that may occur: a mobile medusa and a sessile (fancy term for not
mobile) polyp. Both body forms have tentacles arranged around an opening into the two-layered sac-
like body. The inner tissue layer (derived from endoderm) secretes digestive juices into the
gastrovascular cavity, which digests food and circulates nutrients (doing the job our circulatory AND
digestive systems do). Muscle fibers occur at the base of the epidermal and gastrodermal cells, making
this the first group of muscled animals. Nerve cells located below epidermis near the mesoglea
interconnect and form a nerve net throughout the body. Cnidarians have both muscle fibers and nerve
fibers, making these animals capable of directional movement. The nerve net allows transmission of
messages in more than one direction, possibly an advantage in a radially symmetrical animal, while
contraction of muscle fibers (under control of the nerve fibers) allows for movement. While they have a
nerve net, brains are not present.
Reproduction
Generally speaking Polyps are tube shaped and sedentary with a ring of tentacles around the mouth,
Medusae are umbrella or bell shaped, free living and have a central projection on the inside of the
umbrella which supports the mouth and their tentacles around the rim of the umbrella. In the "typical"
cnidarian life cycle, male and female medusae spawn freely into the sea, where fertilization occurs and
a planula (cnidarian ciliated larva) develops. At metamorphosis, the planula settles on and attaches to
the substratum, where it metamorphoses into a polyp. The primary polyp produces additional polyps
asexually, by budding, stolonic outgrowth, or some other process, to form a clone or a colony. At the
appropriate time, determined perhaps by size of the colony or environmental conditions, rather than or
in addition to polyps, medusae are produced asexually (in Cubozoa, each polyp metamorphoses into a
medusa). They are released to take up a pelagic existence and the cycle begins anew.
Class: Hydrozoa (polypovce)
Hydra are solitary, freshwater hydrozoan polyps. The body is a small tube about one-quarter inch long,
best observed with a dissecting microscope. Four to six tentacles surround the mouth, the only opening
at into the body. Hydra can move from one location by gliding or even somersaulting. Hydras have
both muscular and nerve fibers, and respond to touch. Epidermal cells are termed epitheliomuscular
cells and contain muscle fibers. Cnidocytes and sensory cells are also present in the epidermis.
Interstitial cells can produce an ovary or testis, and may assist regeneration. Gland cells secrete
digestive juices into the gastrovascular cavity into which tentacles have stuffed captured prey.
Digestion is completed within food vacuoles of nutritive-muscular cells. Nutrients diffuse to the rest of
the body. Hydras reproduce both sexually asexually (by budding). In sexual reproduction, sperm from a
testis swim to an egg within an ovary. Following early development within the ovary, a protective shell
forms and allows the embryo to survive until conditions are optimum.
Class: Jellyfish - Scyphozoa (medúzovce)
The body of an adult jellyfish consists of a bell shape producing jelly and enclosing its internal
structure, from which tentacles are suspended. Each tentacle is covered with cells called cnidocytes,
which can sting or kill other animals. Jellyfish lack basic sensory organs and a brain, but their nervous
systems and rhopalia allow them to perceive stimuli, such as light and odor, and respond quickly. They
feed on small fish and zooplankton that become caught in their tentacles. Their digestive system is
incomplete: the same orifice is used to take in food and expel waste. The body of an adult is made up of
94–98% water. Most of the life, they stay in the generative stadium “medusa”. They have different
sexes. From fertilized ovum – planula (ciliated larva), which swims in the water. From planula, which
settles down – polyp and from it with strobilation small jellyfish.
Aurelia aurita (medusa ušatá) – in European seas, 40 cm in diameter.
Cyanea arctica – the biggest jellyfish, 2 m in diameter
Class: Corals - Anthozoa (koralovce)
Sea anemones are solitary polyps 5-100 mm in height and 5-200 mm in diameter or larger. They are
often brightly colored and look like flowers (specifically anemones) on the seafloor. You might
remember them from the film Finding Nemo. The anemone's thick, heavy body rests on a pedal disk
and supports an upward-turned mouth surrounded by hollow tentacles. Sea anemones feed on various
invertebrates and fish. They attach to a variety of substrates, or may be mutualistic with hermit crabs,
living attached to crab's shell.
Metridium senile:
Corals may be solitary but most today are colonial. The majority of corals occur in warm shallow
waters; the accumulation of their calcium-carbonate remains builds reefs. Some corals occur in colder
waters, so the mere presence of coral does not necessarily indicate a tropical environment. Modern
scleractinian coral, dominant reef-builders since the Triassic period (some 230 million years ago), have
symbiotic photosynthetic dinoflagellates living within the coral body. These dinoflagellates are in the
genus Symbiodinium, and are termed collectively zooxanthellae, shown in Figure 13a. Figure 13b
illustrates several living coral tyypes.
Unlike other cnidarians, anthozoans do not have a medusa stage in their development. Instead,
they release sperm and eggs that form a planula, which attaches to some substrate on which the
cnidarian grows. Some anthozoans can also reproduce asexually through budding.
External skeleton of corals includes calcium carbonate, the shells makes coral reefs (the biggest one is
on the northern part of Australia, more then 2 000 km).
Phylum: Acnidaria (rebrovky)
Sea animals, which look like jellyfish. Close to mouth the have few long tentacles, behind mouth they
have gastro vascular system, which makes under the body surface 8 lines. Over them are formations of
cilia, with which they move. In ectoderm they do not have stinging cells, but sticking.
Sources : On-line biology book, Michael J. Farabee
Elncyclopedia of Life, www.eol.org
