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104 Nature

HOW DO STARFISH FEED?

Starfish feed by turning their stomachs inside out over

their victims. They then ooze digestive juices, which

dissolve their prey. Most starfish eat shellfish such as

mussels. They move in search of food using the tiny,

flexible tube feet that protrude from their underside.

4 SEA URCHIN

Sea urchins live on the seabed or

buried in sand. Like starfish, they

have many tiny tube feet, which they

use for crawling and feeding. Sea urchins

are well protected by their many sharp spines.

4 SEA CUCUMBER

These echinoderms can measure

up to 1 m (3 ft 3 in) long. They

live on the sea floor and feed on

decaying matter. After a meal, a

sea cucumber pulls its tentacles

into its mouth to clean them.

The phylum of Echinodermata includes about 6,000 species,

divided into six classes:

• Sea lilies and feather stars • Starfish • Brittle stars and
  basket stars • Sea cucumbers • Sea urchins • Sea daisies

Echinoderms

Starfish, brittle stars, sea urchins, and sea cucumbers

all belong to the family of echinoderms. These

slow-moving, headless invertebrates are the only

animals with bodies based on a five-fold

structure. All echinoderms live in salt water.

HOW DO SPONGES FEED?

Sponges feed by drawing seawater in through pores

in their surface and then filtering out tiny organisms.

Sponges lack the obvious body parts most animals

have. They have no heart or other organs of any

description. Sponges’ bodies are stiffened by tiny

grains of limestone, silica, or a fibre called spongin.

WHAT HAPPENS WHEN A STARFISH LOSES A LIMB?

Starfish that lose limbs can grow new ones in their

place. If a severed limb contains certain cells, it too

can survive and will eventually grow into a whole

new starfish. Losing a limb may help a starfish escape

from a predator’s clutches. Brittle stars are so called

because their limbs break off easily.

YELLOW TUBE SPONGES 3

Sponges exist in a wide variety of

colours and may be shaped like

fingers, chimneys, or vases. They

range in size from less than

10 cm (4 in) to more than

1 m (3 ft 3 in) long.

Sponges

These animals look like plants or fungi, but they are

actually simple invertebrates. Most sponges dwell in salt

water, spending their lives attached to rocks or reefs.

FIND OUT MORE. Invertebrates 102

Tentacles

have evolved

from tube feet

Spines

of some species

are poisonous

Tube feet

pass algae

and animals

to mouth

Anus

discharges

waste

Skeleton

made of tiny

chalky fragments

called ossicles

Mouth

on the underside

of the body

1 SPINY STARFISH

Starfish have a central body

with limbs that radiate

outwards like spokes on a

wheel. Most starfish have

five limbs, although some

have more.

Tiny tube feet

cover the arms –

up to 2,000 in

some species

This phylum includes about 5,000 species, divided into four classes:

• Calcareous sponges • Glass sponges • Demosponges (including the
  familiar bath sponge and tube sponges) • Scelerosponges

SPONGE CLASSIFICATION

ECHINODERM CLASSIFICATION

FIND OUT MORE. Invertebrates 102

echinoderms

sponges