An Invertebrate is an Organism which does not have a Backbone or a Spinal Column. The term Invertebrate does not correspond to a True Taxonomic Class the way the terms 'Bird' or 'Mammal' do. Instead it is an informal term that refers to any Taxonomic Class of Spineless Animals: Sponges, Cnidarians, Flatworms, Molluscs, Arthropods, Insects, Segmented Worms and Echinoderms, to name some of the better known groups. They are classified into 30 Phyla. They range from popular Animals such as Jellyfish, Corals, Slugs, Snails, Mussels, Octopuses, Crabs, Shrimps, Spiders, Butterflies and Beetles to less popular Animals such as Flatworms, Tapeworms, Sipunculids, Sea-Mats and Ticks.
Majority of the Animals found on Earth are Invertebrates. They include 95% of all Animal Species. Of all Invertebrates, the Insects are by far the most numerous. Estimates of the total number of Insects fall in the range of 1 to 30 million. There are also some 10,000 Species of Sponges, 9,000 Species of Cnidarians, 100,000 Species of Molluscs, and 75,000 Species of Arachnids in addition to tens of thousands of Species belonging to other lesser known groups. There are so many Species of Insects that scientists are still searching or are yet to discover them Invertebrates were the first Animals to evolve. Fossil evidence of Invertebrates dates back to the late Precambrian, 600 million years ago. The first life evolved in the form of single cells in water. Invertebrates were the initial few examples of multicellular Organisms that evolved in water. Invertebrates set the path for the evolution of other Organisms as simple transformations started taking place. These simple changes led to complex beings in the form of Vertebrates.
Kingdom - Animalia.
Phylum - Chordata.
They are found all over the World and in variety of Environments, some even live inside other Organisms.
Invertebrates have two basic Body Types:
1. Radial Symmetry - It is a circular body plan arranged around a central mouth, similar to the way spokes radiate out from the hub of a wheel. It includes Animals who spend their adult lives fastened in one place.
2. Bilateral Symmetry - It is a body type in which the right and left halves are similar to each other and they typically have a definite front and back end.
Arachnids - Spiders, Scorpions, Harvestmen, Ticks, Mites etc.
Insects - Bees, Ants etc.
Crustaceans - Crabs, Lobsters, Crayfish, Shrimp, Krill and Barnacles.
Annelid - Ragworms, Earthworms and Leeches etc.
Cnidaria - Sea Anemones, Corals, Sea Pens, Jellyfish, Box Jelly, Portuguese Man o' War etc.
Protozoa - Amoeba, Ciliate etc.
Echinoderms - Starfish, Brittle Stars, Sea Urchins, Sand Dollars, Sea Cucumbers, Feather Stars, Sea Daisies etc.
Molluscs - Snails, Slugs, Squid, Cuttlefish, Octopus Giant Squid etc.
Porifera - Sponge.
Nematode - Round Worms.
Nemertea - Ribbon Worms.
Brachiopoda - Lamp Shells.
Onychophora - Velvet Worms.
Platyhelminthes - Tapeworms, Flatworms, Flukes etc.
Priapulida - Penis Worms.
Rotifer - Wheel Animal.
Sipuncula - Peanut Worms.
Tardigrada - Water Bears or Moss Piglets.