What Are Invertebrates?

By | September 9, 2016

From insects to mollusks, and from worms to jellyfish, invertebrates dominate our planet in terms of their numbers: they make up around 97 percent of the animal kingdom. What features do these animals share in common? Very few! However, they are animals that possess neither a backbone, nor a bony internal skeleton.
Invertebrates are divided into about 30 groups. They include:

ARTHROPODA

 imperial-scorpion

Imperial scorpion

The group Arthropoda includes insects, arachnids (creatures such as spiders and scorpions), and crustaceans. Arthropods make up the largest group of invertebrates, and account for about 90 percent of known animal species.

GARDEN CENTIPEDE

garden-centiped

A centipede has at least 15 pairs of legs. Centipedes are carnivorous, hunting prey such as insects and spiders.

ROSE CHAFER BEETLE

rose-chafer-beetle

There are more than 300,000 species of beetle, some very brightly colored.



MOLLUSCA

 squid

Squids

From a small garden snail to a giant squid, the group that forms Molluska is incredibly varied. Most mollusks have a shell, or at least the remnants of one, but not all—octopuses have no shell, and neither do slugs.

NUDIBRANCH

nudibranch

These marine mollusks are often called “sea slugs.” The infant form (larva) has a shell.

GIANT AFRICAN SNAIL

giant-african-snail

These are large snails—they can reach 8 in (20 cm) in length.

 


ANNELIDA

earthworm

Earthworm

Annelid worms have bodies that are divided into segments. Earthworms and bristleworms are types of annelid worm. Members of this group can be found living in seawater, in fresh water, and on land. Amazingly, there are more than 12,000 recognized species of annelid worm.

RAGWORM

ragworm

These creatures have a pair of swimming legs (called parapods) on each segment of their bodies.

TIGER LEECH

 tiger-leech

Some leeches, like this one, will wait for a passing animal and feed off that animal, sucking its blood.


CNIDARIA

sea-anemone

Sea anemone

This group includes sea jellies (also known as jellyfish), corals, and sea anemones. They have stinging cells called nematocysts. Some can swim, while others remain fixed to the seabed, waiting for food to drift past.

BRAIN CORAL

brain-coral

Many corals are named for their appearance, like this heavily wrinkled coral.

SEA NETTLES

sea-nettles

Jellyfish, such as these sea nettles, have bodies that are largely made of water. Take a jellyfish out of water, and the shape will collapse.

 


ECHINODERMATA

sunflower-sea-star

Sunflower sea star

Many echinoderms have very spiny bodies and nearly all live on the seafloor—none can survive in fresh water. They include starfish (sea stars), sea urchins, and sea cucumbers. Most of these animals can move around, and they have up to 20 legs—but no brain.

SEA CUCUMBER

sea-cucumber

These echinoderms are found on seabeds all over the world.

CROWN-OF-THORNS SEA STAR

crown-of-thorns-sea-star

This is the largest starfish and a voracious predator, feeding on corals. It has needle-sharp spines, each capable of injecting a nasty venom.

 


PORIFERA

Porifera are also called sponges. They were mistaken for plants until the 1700s but, in fact, are very simple animals, with no arms, legs, heads, or sense organs. Sponges have simple baglike or tubelike bodies and live stuck to the seafloor, filtering food from the water.

AZURE VASE SPONGE

azure-vase-sponge

There are thousands of sea sponges, some very colorful.

ELEPHANT EAR SPONGE

elephant-ear-sponge

Some sponge species can grow quite large.

 


TAKE A LOOK—METAMORPHOSIS

metamorphosis

Most invertebrates leave the egg as a larva and undergo several developmental stages before reaching adult form. This is known as metamorphosis.

CATERPILLAR

After hatching from an egg, a butterfly caterpillar proceeds to eat and eat and eat. Its job is to grow quickly.

TIME TO PUPATE

A tough, leathery coat forms around the caterpillar, and it becomes a pupa. After some time, a butterfly will break free.

ADULT FORM

Finally, the butterfly emerges. It has to spread out its wings to dry before it can fly. A butterfly is the adult form.

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