Page 308 - (DK) Ocean - The Definitive Visual Guide

306 ANIMAL LIFE

Echinoderms

THE NAME OF THIS PURELY MARINE group of invertebrates
DOMAIN Eucarya RADIAL SYMMETRY
is derived from the Greek for “hedgehog skin.” The group This tropical starfish has the typical five-
KINGDOM Animalia
includes starfish, sea urchins, brittlestars, feather stars, and sea rayed structure of echinoderms. Its arms
PHYLUM Echinodermata are protected by hard plates, and its bright
cucumbers. Echinoderms have radiating body parts, so most colors warn predators of its toxins.
CLASSES 5
appear star-shaped, disc-shaped or spherical, and all have a
SPECIES About 7,278
skeleton of calcium-carbonate plates under the skin. Inside
is a unique system of water-filled canals, called the
water-vascular system, that enables them to move,
as well as to feed and breathe. Typically bottom-
dwellers, they live on reefs, shores, and the sea bed.

Anatomy
The echinoderm body is based on a five-rayed symmetry similar to the
petals of a flower. This is apparent in starfish, brittlestars, and urchin shells
(tests). Sea urchins are like starfish, with their arms joined to form a ball. Sea
cucumbers resemble elongated urchins —their five-rayed symmetry can be seen
end-on. The echinoderm skeleton is made of hard calcium-carbonate plates, which
are fused to form a rigid shell (as in urchins) or remain separate (as in starfish). Usually,
it also features spiny or knoblike extensions that project from the body. Sea cucumbers
have minimal skeletons reduced to a series of small, isolated
plates. The water-vascular system consists of a network of
canals and reservoirs, as well as tentacles that extend through
pores in the skin to form hundreds of tiny tube feet.
outlet of water-vascular anus
system (madreporite) tube foot
gonad spine
water- calcium-
vascular carbonate
canal plate

intestine

mouthparts
MINI SUCKERS SEA URCHIN BODY PLAN
Tube feet act like hydraulic The body consists of a fluid-filled
suckers. They are operated by cavity inside the shell (test), which
water squeezed in and out from houses the organs. The mouth is in
a small reservoir similar to the bulb the centre of the underside, and the
on the end of an eye dropper. anus is on top of the upper side.

ECHINODERM CLASSIFICATION

The echinoderms are split merging into a central body disc. On the
FEATHER STARS, SEA LILIES SEA URCHINS, SAND DOLLARS
into five classes based on Class Crinoidea underside of the arms are rows of numerous Class Echinoidea
their shape, skeleton, and the tube feet and a groove, along which they
position of their mouth, anus, About 638 species pass food to the central mouth. The mouth About 999 species
Body shape ranges from a disc (sand dollars)
Also known as crinoids, these animals have
and madreporite. A sixth class, a saucer-shaped body extending into five is on the underside, and the anus and to a sphere (urchins) with five double rows
madreporite are on the upper surface.
“the Concentricycloidea,” was repeatedly branching, feathery arms used as The skeleton is a layer of plates (ossicles) of tube feet. The skeletal plates join to form
at one time set up for the newly filter-feeding appendages. Mouth and anus embedded in the body wall. a rigid shell (test) with movable spines.
face upwards. Sea lilies attach to the sea bed
discovered sea daisies but these
by a jointed stalk, but feather stars break free BRITTLESTARS, BASKET STARS SEA CUCUMBERS
two species of disc-shaped
OCEAN LIFE type of starfish and are now STARFISH OR SEA STARS About 2,074 species About 1,716 species
when young to become swimming adults.
Class Holothuroidea
Class Ophiuroidea
animals are in fact a strange
These echinoderms have a disc-shaped body
These echinoderms have a sausage-shaped
regarded as members of the
Class Asteroidea
with five narrow, flexible arms. Basket stars’
body with five double rows of tube feet,
class Asteroidea.
arms are branched and finely divided. The
with those encircling the mouth modified
About 1,851 species
The body of these mostly seabed scavengers is
skeleton is a series of overlapping plates. The
into feeding tentacles. The skeleton
comprises small, multi-shaped plates.
mouth, on the underside, doubles as an anus.
star-shaped, with five or more stout arms