Invertebrates

Invertebrates are animals that do not have a backbone or vertebral column. They make up about 97% of all known animal species on Earth, making them the most diverse group in the animal kingdom.

Learning about invertebrates not only helps students understand biology and nature but also introduces important scientific vocabulary commonly used in English.

Classification of Animals

There are an estimated 4 million animal species in nature. To study and describe this enormous diversity, scientists classify animals into groups based on shared characteristics.

The first major division separates animals into:

• Vertebrates – animals with an internal skeleton and a backbone
• Invertebrates – animals without a backbone

Because invertebrates differ greatly from one another, they are further divided into several major groups.

Main Groups of Invertebrates

Invertebrates are found in almost every environment on Earth and are commonly divided into eight main groups:

• Porifera
• Cnidaria (formerly called Coelenterates)
• Platyhelminthes (Flatworms)
• Nematodes (Roundworms)
• Annelids
• Arthropods
• Molluscs
• Echinoderms 

Porifera (Sponges)

Porifera, commonly known as sponges, are aquatic animals that live attached to surfaces in seas and fresh water.

• Water flows through tiny pores in their bodies
• They obtain oxygen and nutrients by filtering water
• Most reproduce asexually by budding
• Their bodies contain calcareous or siliceous spicules that provide support

Sponges

Cnidaria (Coelenterates)

Cnidarians have a sac-shaped body and stinging tentacles. This group includes:

• Jellyfish
• Sea anemones
• Corals

Coral reefs are formed by millions of tiny polyps connected to one another, creating massive external skeletons such as the Great Barrier Reef in Australia.

Worm-Like Invertebrates

Common Characteristics

• Soft, elongated bodies
• Bilateral symmetry
• Creeping motion (locomotion)

Platyhelminthes (Flatworms)

Flatworms live in fresh or salt water, and some are parasites.

• Flattened bodies
• One digestive opening for both food intake and waste removal
• Breathe through the skin
• Examples: planaria, tapeworms

Nematodes (Roundworms)

Nematodes are cylindrical worms found in many environments.

• Some are free-living
• Others are parasites of plants, animals, or humans
• Examples include pinworms and vinegar eels (not true eels)

Annelids

Annelids have bodies divided into identical segments.

• Live on land, in freshwater, or in the sea
• Examples: earthworms, leeches
• Earthworms are hermaphrodites and feed on decaying organic matter

Arthropods

Arthropods are the largest group of animals, with over one million species.

Key features:

• Jointed legs
• Chitinous exoskeleton
• Segmented body
• Bilateral symmetry

Major Arthropod Groups

Arachnids

• 8 legs
• No wings
• Body divided into cephalothorax and abdomen
• Examples: spiders, scorpions, mites, ticks

Crustaceans

• Mostly aquatic
• Examples: crabs, lobsters, shrimps, woodlice, sea fleas

Myriapods

• Long segmented bodies
• Many legs
• Centipedes (one pair of legs per segment)
• Millipedes (two pairs per segment)

Insects

• Body divided into head, thorax, and abdomen
• 6 legs
• May have wings
• Examples: butterflies, ants, flies, beetles

Molluscs

Molluscs have soft bodies and live in water or on land.

Main Groups

• Bivalves: mussels, clams, oysters (two shells)
• Gastropods: snails and slugs (snails have shells; slugs do not)
• Cephalopods: octopus, squid, cuttlefish

Cephalopods are the most advanced molluscs, capable of fast movement using jet propulsion.

Octopus

Echinoderms

Echinoderms are marine animals with spiny skin.

• Examples: starfish, sea urchins
• Starfish usually have five arms, though some species have many more
• Sea urchins have a spherical body covered with long spines used for movement and defense

Starfish