What are Invertebrates?
The animal kingdom is divided into vertebrates and invertebrates. Vertebrates have a vertebral column (backbone) whereas invertebrates do not. There are about 40,000 species of vertebrates. At least 99% of all animal species are invertebrates, comprising over 30 major groups and over 5 million species. Invertebrates: • include insects, crustaceans (eg. crabs), spiders, coral; • are found in all aquatic and terrestrial environments; and • are usually smaller than vertebrates, but can range in size from microscopic forms to giant squids.
Invertebrates are animals without a backbone. 97% of all animal species are invertebrates, including all worms, insects, arachnids, crustaceans, mollusks, jellyfish, starfish, nematodes, sponges, and all microfauna. The smallest vertebrate, the female Paedocypris fish, is 7.9 mm long, so anything smaller than this is definitely an invertebrate. Genetically, a major difference between vertebrates and invertebrates is that invertebrates have only one cluster of Hox genes, while vertebrates have several. Humans have 4 Hox clusters. All of the 38 animal phyla except for a subset of one, Chordata, are invertebrates. Vertebrates and invertebrates split evolutionarily between 525 and 520 million years ago, when eight early vertebrate animals appear in the fossil record, most notably the jawless fish Myllokunmingia. Prior to this, animals existed for at least 80 million years in an exclusively invertebrate form. Since then, vertebrates have been the world’s most successful animals in terms of