Characteristics of the Invertebrate Groups

The Arthropods

The arthropod group includes all invertebrates which have a skeleton on the outside of their bodies. The exoskeleton is made of chitin and the legs are jointed. This group is so large that it has been divided again into four sub-groups - insects, crustaceans, spiders and myriapods.

Insects

All insects have three parts to their body - the head, the thorax and the abdomen. The insects have three pairs of legs attached to the thorax. Some insects, such as the dragonfly, have two pairs of wings. Others, such as the housefly, have only one pair of wings. A few insects, such as the flea, have no wings at all.

Crustaceans

The body is made up of many segments. There are antennae on the third segment and jaws (mandibles) on the fourth segment. Most crustaceans have a hard, protective carapace on their backs. Examples of crustaceans are the shrimp, the lobster and the woodlouse.

Spiders

All spiders have a head with eight eyes. The other part of the body is the abdomen which is usually larger than the head. They all have four pairs of legs.

Myriapods

The myriapod sub-group is represented by millipedes and centipedes. Millipedes are herbivores while centipedes are carnivores. The millipede has two pairs of legs attached to each segment of its body. It does not really have 1,000 legs! In fact millipedes can only have up to 710 legs. The centipede only has one pair of legs per body segment. A centipede may have between 28 and 354 legs.

Other Invertebrate Groups

Jellyfish

All jellyfish have a jelly-like body. They are all aquatic. They have tentacles which are armed with many stinging cells. When a prey is caught in the jellyfish's tentacles, it is stung to stop it struggling. The tentacles then move the prey to the mouth. The parts of the prey which cannot be digested are eliminated through the mouth, which is the only opening to the body. Examples of jellyfish are the sea anemone and the Portuguese man-o'-war.

Starfish

All starfish live in the sea. Their skin is hard because it contains a chalky substance. They also have spines over the surface of their skin. Starfish have five 'arms' with many suckers on them. The sea urchin also has five 'arms' but these have joined together to form a round body.

Worms

Ringed worms, also called true worms, have long, segmented bodies with a definite head at their anterior end. They have short, stiff hairs, called bristles, in rows along the sides of their bodies. Most of these worms burrow in the sand or in the earth. Examples of true worms are the ragworm and the earthworm.
Flatworms have flat bodies. Many flatworms, such as the tapeworm and the liver fluke, are parasites.
Roundworms are long, thin and pointed at both ends. Many roundworms are also parasites. Some infest in the human gut. Although each one is only 20cm long, an infected person may have hundreds of them in the small intestine!

Johnny's Fact File No.5 The largest invertebrate is found in the sea. It is the giant squid which can reach a length of up to 17 metres and  a mass of 2 tonnes. It can reach speeds of up to 56 km/h.

Molluscs

Molluscs have a muscular foot and a shell. Sometimes the shell is inside the body, as in the slug. Some aquatic molluscs have tentacles, such as the octopus and the squid. Most land molluscs have a radula, a tongue with many rows of tiny teeth.

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