Invertebrates: Arthropods, Mollusks, and Annelids
Invertebrates are animals without a backbone, and they make up the vast majority of animal species, including arthropods, mollusks and annelids. In brief: arthropods (insects, spiders, crabs) have jointed legs and an external skeleton, mollusks (snails, clams, octopuses) have a soft body usually protected by a shell, and annelids (earthworms, leeches) have bodies divided into ring-like segments. A butterfly, a garden snail and an earthworm are three different invertebrates.
Roughly nine out of every ten known animal species are invertebrates. This guide describes the features of three major invertebrate phyla, separates the insects from the arachnids, compares the groups in a table, shows how to identify a specimen in a worked example and explains the mistakes people often make.
What are invertebrates?
Invertebrates are all animals that lack a vertebral column. This is not a natural evolutionary group but a convenient label for many separate phyla, including sponges, cnidarians, flatworms, roundworms, mollusks, annelids, arthropods and echinoderms.
They live in nearly every habitat, from deep sea vents to deserts and soil. Their body sizes range from microscopic to giant squids that may exceed ten meters in total length. Because they have no internal bony skeleton, they support their bodies with a hard outer covering, a shell or fluid pressure.
What are arthropods?
Arthropods are the largest animal phylum. Their name means “jointed foot”. They share three main features: a hard external skeleton, called an exoskeleton, made of chitin, jointed legs and a segmented body.
The exoskeleton protects the animal and prevents water loss, but it cannot grow. Arthropods therefore molt, shedding the old skeleton and growing a new one. They have open circulatory systems, in which blood bathes the organs, and a nervous system with a brain and a ventral nerve cord.
The main groups include insects, arachnids, crustaceans and myriapods. Crustaceans such as crabs, shrimp and woodlice usually have two pairs of antennae and live in water or damp places. Myriapods, which include centipedes and millipedes, have many legs and long bodies.
How do insects differ from arachnids?
Insects
An insect has three body parts: head, thorax and abdomen. It has six legs, all attached to the thorax, one pair of antennae and usually two pairs of wings. Beetles, ants, flies, bees and butterflies are insects. Insects are the most species-rich group of animals known.
Arachnids
An arachnid has two body parts: a cephalothorax, which joins the head and thorax, and an abdomen. It has eight legs, no antennae and no wings. Spiders, scorpions, ticks and mites are arachnids. Most spiders produce silk and inject venom to subdue prey.
What are mollusks?
Mollusks have a soft, unsegmented body. Most have a muscular foot for movement, a mantle that makes the shell, and a visceral mass containing the organs. Many species feed using a rasping tongue called a radula.
- Gastropods: Snails and slugs with a single shell or none, which move on a flat foot.
- Bivalves: Clams, mussels and oysters with two hinged shells. Most filter food from water.
- Cephalopods: Octopuses, squid and cuttlefish, which have tentacles or arms, a well-developed brain and, in most, a closed circulatory system. They are active predators with excellent eyes.
What are annelids?
Annelids are segmented worms. Their bodies are divided into repeating ring-shaped segments, visible as rings on an earthworm. They have a true coelom, a closed circulatory system and a complete digestive tract.
Earthworms burrow in soil, and their tunnels aerate the ground. Many marine annelids have bristles for swimming or crawling, and leeches have suckers at both ends of the body. Annelids breathe through their moist skin, so they need damp surroundings.
Invertebrate groups compared
| Group | Skeleton or covering | Body segments | Legs or limbs | Example |
|---|---|---|---|---|
| Insects | Chitin exoskeleton | 3 parts | 6 legs | Ant |
| Arachnids | Chitin exoskeleton | 2 parts | 8 legs | Spider |
| Crustaceans | Hard exoskeleton | 2 or 3 parts | Usually 10 or more | Crab |
| Mollusks | Shell or none | Not segmented | Foot, arms or tentacles | Snail |
| Annelids | Soft body with a cuticle | Many rings | None, bristles in some | Earthworm |
Worked example: identifying a specimen
A student finds a small animal in the garden. It has a hard body, three clear body regions, six legs and two antennae. Use the table and the questions below.
- Backbone? No visible backbone, and the body is covered by a hard shell-like layer. It is an invertebrate.
- Jointed legs? Yes, so it is an arthropod.
- Count the legs: Six legs. Arachnids have eight and myriapods many more, so this points to an insect.
- Confirm with body parts: Head, thorax and abdomen fit an insect.
The animal is an insect. A different specimen, soft and slimy with a single spiral shell and a muscular foot, would be a gastropod mollusk. A long, soft worm with visible rings would be an annelid. The method is a sequence of simple tests, each of which removes some possibilities.
How did invertebrates evolve?
Animal fossils from the Cambrian period, about 540 million years ago, include early arthropods, mollusks and annelid-like worms. This rapid appearance of many body plans is often called the Cambrian explosion, though evidence suggests that some animals were present earlier.
Insects arose later, probably in the Devonian period, and were among the first animals to fly. The Carboniferous period had giant dragonfly-like insects with wingspans of about 70 centimeters. Scientists link this size partly to higher oxygen levels, but the explanation is still debated.
Why are invertebrates important?
Invertebrates pollinate crops, recycle nutrients and form the base of many food webs. Earthworms improve soil structure, bivalves filter water, and bees pollinate a large part of the fruits and vegetables people eat.
Some are harmful: mosquitoes carry diseases, and locusts and beetles destroy crops. Invertebrates are also used in research, for example fruit flies in genetics and squid axons in studies of nerve signals. Declines in insect numbers reported in some regions are a concern for scientists.
Which mistakes are common?
Mistake 1: Calling spiders insects
Spiders have eight legs and two body parts, so they are arachnids. Insects have six legs and three body parts.
Mistake 2: Thinking worms are one group
Earthworms are annelids, flatworms are a separate phylum, and roundworms are another. “Worm” describes a body shape, not a single group.
Mistake 3: Believing that all mollusks have shells
Slugs have almost no shell, and an octopus has none. The common features are the mantle, the foot and, in most, the radula.
Mistake 4: Saying that invertebrates are simple or primitive
Octopuses solve problems, bees communicate the location of flowers and ants build complex colonies. Having no backbone does not mean having a simple body or behavior.