Category: Invertebrates
These ocean dwelling animals are characterized by bilateral body symmetry, a prominent head, and a set of arms, or tentacles. There are about 800 identified species of cephalopods, a class that includes squid, octopi, nautili, and cuttlefish. With their well-developed senses and large brains, cephalopods are widely considered the most intelligent of the invertebrates. With only a few exceptions, all cephalopods have an ink sac, which can be squirted into the surrounding water to confuse predators. They are also the only mollusks with a closed circulatory system.
The world’s first Animals
Invertebrates were the first animals to evolve. While their soft bodies left precious few records behind, there are fossils of sponge-like animals that are 665 million years old, as well as traces of tracks and burros from worm-like animals going back 1 billion years. By comparison, the earliest birds appeared only 150 million years ago! And not only were invertebrates first, they are the most plentiful of animals - it is estimated that 97% of all animals living on the Earth today are invertebrates, with several groups containing more species than all vertebrates combined!
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