Page 262 - (DK) Ocean - The Definitive Visual Guide

260 ANIMAL LIFE

Cnidarians HUMAN IMPACT
CORAL TRINKETS

THIS ANCIENT GROUP OF AQUATIC ANIMALS emerged in Many corals are harvested for sale
DOMAIN Eucarya as souvenirs, and the most valued
Precambrian times, about 600 million years ago. It includes species are being overcollected.
KINGDOM Animalia
reef-building corals, anemones, jellyfish, and hydroids, most Particularly desirable are certain
PHYLUM Cnidaria soft corals, in which the calcareous
of which are marine. Cnidarians have a radially symmetrical
CLASSES 5 supporting column is so strong
body shaped like a simple sac, with stinging tentacles around a and dense it can be carved and
SPECIES 10,886 polished. They include the red or
single opening that serves as both mouth and anus. There are precious coral, Corallium rubrum
two body forms: the polyp form, typified by sea anemones, which is fixed to a solid (below). As yet, there are no
international regulations
surface and has an upward-facing mouth and tentacles; and the medusa, shown by controlling trade in this species,
adult jellyfish, which can swim and has a downward-facing mouth and tentacles. although some countries restrict
its collection. Black corals (order
Antipatharia) also have strong
skeletons that can be carved.
Anatomy
Corals and anemones exist only as polyps, whereas other cnidarians
can be either polyps or medusae at different stages of their life cycle. The
body wall of both polyps and medusae consists of two types of tissue. On
the outside is the epidermis, which acts like a skin to protect the animal.
The inner tissue layer, lining the body cavity, is the gastrodermis, which
carries out digestion and produces reproductive cells. Separating and
connecting these two layers is a jellylike
substance called the mesoglea. The
tentacles have stinging cells called
cnidocytes, which are unique
TENTACLE ARRANGEMENT
The number of tentacles on coral to this phylum and give
polyps varies from one group to it its name. A simple
another. The polyps of all soft corals nervous system
(above) have eight tentacles, hence
their alternative name of octocorals. responds to touch,
Hexacorals (right) have tentacles chemicals, and
arranged in multiples of six. temperature.

POLYP mesoglea
tentacle POLYP AND MEDUSA epidermis
Polyps are essentially gastrodermis
cnidocyte a tube, closed at one gut
epidermis end, that attaches to a mouth
mesoglea hard surface by a basal shelf of
gastrodermis disk. They live singly or muscle
in colonies. Medusae (velum)
budding are bell-shaped and
juvenile
usually have a thicker
gut mesoglea; some also
basal have a shelf of muscle
disk for locomotion. tentacle
MEDUSA
epidermal cell
coiled thread
STINGING CELLS
BEFORE
Each cnidocyte contains a bulblike
nematocyst DISCHARGE
structure, called a nematocyst, which
barbs
houses a coiled, barbed thread. When
triggered by touch or chemicals, the
thread explodes outward and pierces
the prey’s skin. The animal’s tentacles
uncoiled AFTER
hollow thread DISCHARGE are then used to haul the victim in.
SCLERITES
Small slivers of calcium
carbonate called sclerites
are scattered through the
tissues of soft corals and
sea fans. Here, they are
visible as white shards
under the skin of this
OCEAN LIFE BUILDING REEFS
soft coral.
Coral reefs are built by colonies of coral
polyps that secrete a hard exoskeleton
of calcium carbonate. As the tiny polyps
divide and grow, the reef expands.