2. Radiate Animals
■ Radiata or Radiates is a historical
taxonomic rank that was used to classify
animals with radially symmetric body
plans. The term Radiata is no longer
accepted, as it united several different
groupings of animals that do not form a
monophyletic group under current views
of animal phylogeny. The similarities once
offered in justification of the taxon, such as
radial symmetry, are now taken to be the
result of either incorrect evaluations by
early researchers or convergent evolution,
rather than an indication of a common
ancestor. Because of this, the term is used
mostly in a historical context.
3. Phylum Cnidaria
■ Phylum Cnidaria, is an interesting group
of more than 9000 species. It takes its
name from cells called cnidocytes, which
contain the stinging
organelles (nematocysts) characteristic
of the phylum. Nematocysts
are formed and used only by cnidarians.
Another name for the phylum,
Coelenterata, is used less commonly
than formerly, and it sometimes now
refers to both radiate phyla, since its
meaning is equally applicable to both.
4. There are four major groups of cnidarians:
Anthozoa, which includes true corals, anemones, and sea
pens;
Cubozoa, the amazing box jellies with complex eyes and
potent toxins;
Hydrozoa, the most diverse group with siphonophores,
hydroids, fire corals, and many medusae;
Scyphozoa, the true jellyfish.
5. Characteristics
■ Entirely aquatic, some in fresh water but mostly marine
■ Radial symmetry or biradial symmetry around a longitudinal axis
with oral and aboral ends; no definite head
■ Two basic types of individuals: polyps and medusae
■ Exoskeleton or endoskeleton of chitinous, calcareous, or protein
components in some
■ Body with two layers, epidermis and gastrodermis, with
mesoglea (diploblastic); mesoglea with cells and connective tissue
(ectomesoderm) in some (triploblastic)
■ Gastrovascular cavity (often branched or divided with septa) with a single
opening that serves as both mouth and anus; extensible tentacles usually
encircling the mouth or oral region
6. ■ Special stinging cell organelles called nematocysts in either epidermis or
gastrodermis or in both; nematocysts abundant on tentacles, where they may
form rings
■ Nerve net with symmetrical and asymmetrical synapses; with some sensory
organs; diffuse conduction
■ Muscular system (epitheliomuscular type) of an outer layer of longitudinal
fibers at base of epidermis and an inner one of circular fibers at base of
gastrodermis; modifications of this plan in higher cnidarians, such as separate
bundles of independent fibers in the mesoglea
■ Asexual reproduction by budding (in polyps) or sexual reproduction by
gametes (in all medusae and some polyps); sexual forms monoecious or
dioecious; planula larva; holoblastic indeterminate cleavage
■ No excretory or respiratory system
■ No coelomic cavity
8. Food and Feeding
■ All cnidarians are carnivores. Most of them use their cnidae and the associated toxin to
catch food, although in fact none of them pursue prey. Sedentary polyps depend on food
from organisms that come into contact with their tentacles. Some, such as colonial corals
with the smallest polyps, feed on dispersed material collected in the mucus, which is
absorbed into the mouth by cilia. The hydromedusa alternately floats up and sinks: on the
ascending path its tail tentacles are not inclined to collide with food organisms, but when
submerged, the elongated tentacles “catch fish” through the water, capturing food. After the
product has been captured, the tentacles move it towards the mouth, bending in this
direction or passing it to the tentacles closer to the mouth. The mouth opens, the lips seize
the food, and the muscular actions completely swallow.
■ The edges of the mouths of some scyphomedusae are elaborated into mouth arms that trail
behind the slowly swimming jellyfish, presenting huge surfaces for food gathering. The
mouth of a scyphomedusa of the order Rhizostomae is subdivided into thousands of minute
pores that lead by tubes to the coelenteron. Each pore is associated with an external ciliated
gutter that collects minute organisms and detrital material as the medusa rests mouth-
upward on the sea bottom.
9.
10. Coral reef
■ A coral reef is an underwater ecosystem characterized by reef-building corals. Reefs are
formed of colonies of coral polyps held together by calcium carbonate. Most coral reefs are
built from stony corals, whose polyps cluster in groups.
■ Coral belongs to the class Anthozoa in the animal phylum Cnidaria, which includes sea
anemones and jellyfish. Unlike sea anemones, corals secrete hard carbonate exoskeletons
that support and protect the coral. Most reefs grow best in warm, shallow, clear, sunny and
agitated water.
■ Often called "rainforests of the sea", shallow coral reefs form some of Earth's most diverse
ecosystems. They occupy less than 0.1% of the world's ocean area, yet they provide a
home for at least 25% of all marine species, including fish, mollusks, worms, crustaceans,
echinoderms, sponges, tunicates and other cnidarians. Coral reefs flourish in ocean waters
that provide few nutrients. They are most commonly found at shallow depths in tropical
waters, but deep water and cold water coral reefs exist on smaller scales in other areas.
11.
12. Importance
■ Prominent among organisms that foul water-borne vessels are sedentary cnidarians,
especially hydroids. The muscles that make scyphomedusae strong swimmers are dried
for human consumption in Asia. Sea anemones are eaten in some areas of Asia and
North America.
■ Throughout the tropics where reefs are accessible, coral skeletons are used as building
material, either in blocks or slaked to create cement. Another use for cnidarian
skeletons is in jewelry. The pink colour known as “coral” is the hue of the skeleton of a
species of hydrocoral. Other hydrocorals have purplish skeletons. Skeletons vary in hue,
and those considered most desirable command a high price. The core of some sea fans,
sea whips, and black corals are cut or bent into beads, bracelets, and cameos.
■ All cnidarians have the potential to affect human physiology owing to the toxicity of
their nematocysts. Most are not harmful to humans, but some can impart a painful
sting—such as Physalia, the Portuguese man-of-war, and sea anemones of the genus
Actinodendron. These, and even normally innocuous species, can be deadly in a massive
dose or to a sensitive person, but the only cnidarians commonly fatal to humans are the
cubomedusae, or box jellyfish. Extracts of many cnidarians, mostly anthozoans, have
heart-stimulant, antitumour, and anti-inflammatory properties.
