By far the majority of animals are small and have no backbone. These are called invertebrates—animals without backbones.
Insects alone number over 1,000,000 species. They swarm everywhere, and live on the highest mountains and down the deepest mines. They can be found in the ground, on bushes and trees, in rivers, ponds and lakes; some even in deepest snow and others in hot springs. Insects spread right down to the sea shore and only then are they halted. They are not sea creatures. Even so there are one or two water bugs which manage to live on the ocean surface.
Not surprisingly, insects can be destructive and harmful to man. They invade crops and attack farm animals. They kill garden plants and do damage in our homes. We may even suffer in our bodies from their unwanted attentions. Some insects cause disease, such as the mosquito which carries the malaria germ.
Insects are readily recognised by having six legs, a body divided into three parts—head, thorax and abdomen—and most are winged. Also, they pass through stages that progress, with some exceptions, from an egg to a larva which feeds and grows, shedding its outer skin from time to time. The larva then changes into a pupa and finally into an adult creature. The adult does not grow and usually lives a short life. Insects like grasshoppers and dragonflies pass direct from larva, called a hopper or nymph, into an adult.
Arachnids
Arachnids are invertebrates which are sometimes confused with insects. They consist of spiders, scorpions and mites. In their case the combined head and thorax is joined to the abdomen. The adult arachnid has eight legs and biting jaws. Scorpions have elongated bodies and a sting on the 'tail'. Mites are small and occur in the ground, on plants and animals, and can be harmful.
Millipedes
Soil is the main home of another group of invertebrates, the millipedes and centipedes, which have long bodies and numerous legs. Millipedes are plant-eaters and have rounded bodies and a double pair of legs to each segment. Centipedes have biting jaws, a flattened body, and are hunters. There is a single pair of legs to each segment.
Crustaceans
Some invertebrates live in the sea. These are the crustaceans. They have a hardened shell-like body, and many pairs of legs which operate in different ways. They may be used for walking, feeding, swimming, catching food, or even for breathing. The best known crustaceans are the lobster, shrimp and crab, but there are a great number of smaller kinds. Some live in freshwater, like the water flea, Daphnia, and many tiny creatures in the sea drift about as plankton.
Molluscs
Molluscs include the familiar slug, snail and mussel. As many live in the sea as on land. Some are naked, others have a shell. The shell can be single, as in the snail, which is called a univalve. The mussel with its hinged double shell is known as a bivalve. Looking very different, yet still molluscs, are the octopus, squid and cuttlefish, which all have tentacles. These take the place of the foot which a snail uses for crawling.
Worms
What is usually known as a worm could mean anything which is long and crawls. There are a number of 'worm' groups. The well-known earthworm belongs to a group called Annelida (LatÃn annulus—a ring). Its body is divided into segments. There are many kinds living in tubes in the mud and sand on the sea bottom. Fishermen dig them up for bait. Some have fan-shaped gills which are used to breathe with and to comb the water for food. Roundworms, or nematodes, on the other hand, have no segments and are pale in colour. They live in darkness, either in the soil or as parasites in plants and animals, including man. A person suffering from 'worms' has probably become a victim. Eggs which pass through the mouth develop into nematodes which settle in the intestine, and cause weakness by stealing food. Their eggs then pass out. Clean habits and proper sanitation help to avoid this kind of illness. There are about 10,000 species of roundworms.
Flatworms, which belong to the phylum Platyhelminthes, are of two kinds. Some, called flukes, are leaf-shaped and again are often parasitic. Here, the life story is more complicated. A well-known example is the liver fluke of sheep. A swimming larval fluke attacks a water snail, changes its form many times, then swims away to settle on a blade of grass at the water's edge. A sheep eats the grass and the larva ends up in its liver to grow up. The mature fluke lays eggs which pass out as water larvae, and these seek out more snails. This is why liver fluke disease usually occurs in chalk country where there are more snails.
Tapeworms are long, ribbon-like flatworms which live inside animals, sometimes in humans. They are fixed to the wall of the intestine by a tiny head which has suckers or hooks on it. The tape is in segments, and the ripe ones at the end of the tape break away to release the eggs. These may be picked up by a smaller animal and will turn into a larval tapeworm. If this smaller animal is then eaten by the larger one a tapeworm will result. In the case of the pork tapeworm the pig swallows an egg, and a larval worm forms in its flesh. A human may then eat this pork and swallow the larval, which then turns into a tapeworm. This is why meat inspection and proper cooking of meat is important so as to make it safer to eat. There are about 6,000 species of flatworms.
Other Groups
Other groups of invertebrates include the coelenterates (anemones and jellyfishes), sponges and rotifers.
