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106 Nature

FIND OUT MORE. Invertebrates 102

WHAT FEATURES DO MOLLUSCS HAVE IN COMMON?

As well as a shell, most molluscs have a muscular foot

for creeping or burrowing. Some also have a head with

sense organs. The soft body includes lungs or gills for

breathing, and digestive and reproductive parts, all

enclosed by a skin-like organ called the mantle.

There are about 51,000 mollusc species split into several major classes:

• Gastropods (the largest class) include slugs, snails, winkles, whelks, and
  limpets • Bivalves include scallops, clams, and oysters • Squid, octopuses,
  cuttlefish, and nautilus are cephalopods • Smaller groups include tusk
  shells and chitons (oval molluscs with jointed plates)

MOLLUSC CLASSIFICATION

HOW DO MOLLUSCS FEED?

Most molluscs have a rasping tongue called a radula,

armed with tiny teeth. This scrapes tiny plants and

animals off rocks or tears food into chunks. Bivalves,

such as oysters and mussels, filter food particles from

the water with their gills.

HOW DO MOLLUSCS REPRODUCE?

Molluscs reproduce sexually. Slugs and snails are

hermaphrodite (possessing both male and female

organs), but they must still mate to fertilize their eggs.

Most aquatic molluscs lay eggs that hatch into small,

free-swimming larvae called veliger.

WHAT ARE MOLLUSC SHELLS MADE OF?

Mollusc shells are made of a chalky material called

calcium carbonate. The shell has three layers for extra

strength: a tough outer layer, a chalky middle layer,

and a shiny inner layer, next to the animal’s skin.

The shiny layer in some bivalve molluscs is known

as mother-of-pearl.

NAUTILUS SHELL 3

The nautilus is a cephalopod (a relative of squid

and octopuses) with a many-chambered shell.

The mollusc lives only in the shell’s largest

outer chamber. The smaller inner chambers

are used to control the animal’s buoyancy.

The spiralling form can be seen clearly in

this cut-away shell.

Mollusc shells come in many shapes and

sizes, but most have the same, simple

function – providing somewhere to hide in

times of danger. In land molluscs, the shell

also helps to prevent the moist, soft-bodied

creature from drying out.

HOW DO MOLLUSCS MAKE SHELLS?

A mollusc’s mantle (skin) releases

liquid shell materials, which harden on

contact with water or air. Gastropod and nautilus shells grow

from their outermost edge. As the mollusc grows, its shell develops

more whorls (single turns in a spiral shell) or chambers. In bivalves, new

shell material is deposited on the edge that is farthest from the hinge.

Tentacles

coordinated by a

powerful brain

Sensors on the

arms taste what

they touch

Eyes, similar

to human

eyes, give

excellent

vision

Suckers allow

the octopus to

grab hold of

slippery prey

QUEEN SCALLOP 1

Like all bivalves, the queen scallop has a two-part shell. Most bivalves live

attached to rocks or in burrows on the seabed. They take in water using a

muscular tube called a syphon, and remove food particles with their gills.

SHELLS

2 OCTOPUS IN ACTION

Cephalopods such as octopuses

and cuttlefish are stealthy

hunters. Octopuses creep along

the seabed or lie in wait for fish

and crabs. They pounce on their

prey, seize it with their suckered

arms, and paralyse it with

poisonous saliva.

Muscles hold shell

open for feeding

Eyes along the

edge of the shell

look out for predators

Shell can snap

shut at the first

sign of danger

Molluscs

Soft-bodied invertebrates, molluscs

include slugs, snails, octopuses, squid,

clams, and mussels. Most molluscs

have. SHELLS to protect them.

1 GIANT AFRICAN SNAIL

Gastropods such as snails

glide along on a trail of slime

oozing from an area by their

broad, muscular foot. The

distinct head often carries two

pairs of tentacles. The longer

set may bear eyes on the tips.

Gastropods live mainly in

water, but also in a variety

of land habitats.

Buoyancy chamber

Outer chamber contains the living animal

shells