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.
Geographical Range and Habitat
They are found all over the World and in variety of Environments, some even live inside other Organisms.
Characteristics of Invertebrates
Absence of Backbone - This is the most common and profound physical feature of Invertebrates.
Small Size - Due to lack of a supportive system, majority of the Invertebrates are small and only a few reach big size.
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.
Simple Nervous System - Most Invertebrates have Simple Nervous Systems and they behave almost entirely by instinct. They are incapable of learning from their mistakes. Moths, for example, repeatedly flutter around bright lights, even at the risk of getting burned. Notable exceptions are Octopuses and their close relatives, which are thought to be the most intelligent Animals in the Invertebrate world.
Cold Blooded - All Invertebrates are Cold Blooded, which means that they regulate their body temperature by Thermoregulation. Their body temperature depends on the temperature of their environment.
Multicellular - They are multicellular. It's more than being a colony of individual cells. The cells are working together for the survival of the organism. All of the cells have specific duties and responsibilities.
Absence of Cell Walls - Invertebrates don't have Cells with a Wall.
Absence of Internal Skeleton - Invertebrates do not have a Bony or Internal Skeleton. For example, Sea Anemones have a Hydrostatic Skeleton that produces support via sheets of muscles and an internal cavity filled with fluid. Other Invertebrates such as Insects and Crustaceans have a hard Outer Shell or Exoskeleton.
Sexual Reproduction - Most of them reproduce sexually, not asexually.
Heterotrophic - It means that Invertebrates can't make their own food. Heterotrophs feed off other Organisms to get their energy.
Some Major Types of Invertebrates
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.