Almost every higher animal species on Earth falls into one of two categories, either vertebrates or invertebrates. Vertebrates are creatures that have a backbone or spinal column, and invertebrates are creatures that do not.
Vertebrates tend to be more well-known because most land dwelling creatures are vertebrates, including mammals, birds, reptiles and amphibians. Invertebrates are perhaps the most interesting scientifically because many are some of the oldest living creatures from an evolutionary perspective.
The seven primary groups of invertebrates are:
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- Sponges - The most ancient of all animal phylum, sponges tend to live in saltwater but there are some notable exceptions.
- Ctenophora - Ctenophores are very similar to cnidarians; they usually have soft gelatinous bodies. There are very few Ctenophora, but they include the sea gooseberry, comb jellies and sea walnuts.
- Cnidaria - Another ancient group of animals, Cnidaria usually have stinging tentacles and can reproduce asexually. Cnidaria were grouped with Ctenophora for a long time but were placed in separate phyla after scientists discovered new differences between the groups. Well-known cnidarians include jellyfish and corals
- Echinoderms - Echinoderms notably have external skeletons. They include starfish and sea urchins.
- Worms - Worms live in many different environments. They include regular earthworms, flatworms and roundworms.
- Mollusks - Mollusks are soft creatures that usually reside inside a shell. The most common include mussels, oysters, clams and snails.
- Arthropods - Arthropods are easily the largest group of animals on Earth thanks to the inclusion of insects. They also include arachnids, centipedes and crustaceans.