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Definition: invertebrate from The Penguin Dictionary of Science

General term of convenience given to an animal species that is not a member of the ➤chordate subphylum Vertebrata. In modern classification systems the term has no taxonomic status. ➤➤Appendix table 8.

Summary Article: What is an Invertebrate?
From Animals: A Visual Encyclopedia

Invertebrates are animals that have neither a backbone nor a bony internal skeleton. This group makes up more than 95 percent of the animal kingdom. It is incredibly varied. Some invertebrates are little known, like the microscopic rotifers (animals that may be smaller than bacteria). Others are more familiar. Snails and spiders, fleas and flatworms, centipedes and corals: these are all invertebrates. There are simple invertebrates, such as sponges, that have no brain or internal organs. And there are complex ones, like the highly intelligent octopus.


An estimated five million species of invertebrates exist today. This could double as we learn more about the habitats of invertebrates.

  • Nematodes, or roundworms, are possibly the most numerous creatures on Earth. Some are so tiny that as many as 90,000 can be counted on one rotting apple.

  • Invertebrates sometimes gather together in huge numbers. One of the biggest locust swarms on record contained 72,000,000,000 insects and covered 463 sq miles (1,200 km2).

  • Octopuses have shown that they are brainy. A female octopus in a German zoo watched keepers unscrewing the lids of jars of shrimps—and learned how to do it herself.

Invertebrates from the past

One of the first invertebrate groups to appear on Earth were the sponges, some 600 million years ago. Fossils of soft-bodied invertebrates, like jellyfish, are rare. However, fossils of trilobites, crustaceanlike arthropods, are abundant. Trilobites survived for 300 million years, becoming extinct about 250 million years ago. Other fossil finds include giant griffonflies with 30 in (75 cm) wingspans, water-scorpions 61/2 ft (2 m) long, and giant marine mollusks with shells 30 ft (9 m) long.

Living on others

Among the invertebrates are most of the world’s parasites. These are animals that live on the outside or the inside of other animals, including humans. Many, though not all, are harmful. Common parasites include worms of various kinds that live in their host’s intestines. Pests such as warble flies lay eggs in the hair of mammals like horses and cattle. When the grubs hatch, they burrow into the skin, causing sores. Some insects lay their eggs on other insects, and the emerging grubs eat their host.

Creating new life

Invertebrates reproduce in various ways. Not all of them need to find a mate. Sponges and starfish can create new individuals from bits of their own bodies. In fact, if two sponges are put through a sieve and mixed up, they clump together to form a single animal. Many insects lay unfertilized eggs that hatch and develop into replicas of their parent. Stick insects, water fleas, and aphids all reproduce like this.

Copyright © 2008 Dorling Kindersley Limited

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