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:
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.
ROSE CHAFER BEETLE
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.
GIANT AFRICAN SNAIL
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.
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.
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.
CROWN-OF-THORNS SEA STAR
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
ELEPHANT EAR SPONGE
TAKE A LOOK—METAMORPHOSIS
Most invertebrates leave the egg as a larva and undergo several developmental stages before reaching adult form. This is known as metamorphosis.
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.
Finally, the butterfly emerges. It has to spread out its wings to dry before it can fly. A butterfly is the adult form.