spinal cord
Some invertebrates evolved a rigid rod along the length of their body. This rod is called a notochord. You can see the notochord in the tunicates in Figure below . The notochord gives the body support and shape. It also provides a place for muscles to attach. It can counterbalance the pull of the muscles when they contract. Animals with a notochord are called chordates. All of them are placed in Phylum Chordata. Some early chordates eventually evolved into vertebrates.
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