Page 19 - Science Course 2 (Book 1)

Mo1-L2a: What is an Invertebrate?

Mollusks • Like a roundworm, an arthropod has a hard outer
covering, so it must molt in order to grow.
• Snails and slugs are mollusks in phylum Mollusca.
Antenna
Head
Thorax

Leg
Abdomen
• A mollusk has a mass of tissue called a mantle—a Wings
thin layer of tissue that covers a mollusk’s internal
organs.

Nephridum Heart
Coelom Gonads
Mantle Intestine
Stomach
Mantle Cavity
Shell
Anus Radula

Gill
Mouth
Foot Nerve Cords
Esophagus • Arthropod bodies have three parts: a head, a thorax,
and an abdomen.
• A mollusk’s shell supports and protects its soft
body, though some mollusks, such as slugs, do not • Most arthropods are insects.
have shells.

Roundworms

• Animals in phylum Nematoda are called nema • One trait of insects is metamorphosis, in which the
todes or roundworms. body form of an animal changes as it grows from
an egg to an adult.

• Roundworms have a hydrostatic skeleton for
movement and a hard outer covering, called a
cuticle, for protection.

• An outer covering is shed
and replaced in a process
called molting.
• There are three other major groups of arthropods:
spiders and scorpions, crabs and lobsters, and
centipedes and millipedes.

Arthropods

• There are more animals in
phylum Arthropoda than in
all other animal phyla combined.

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