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Determining whether a creature is a vertebrate or invertebrate may not be something you do on a daily basis, but it's worth knowing which creatures are which. The major difference is a backbone: invertebrates lack this bone structure. Spiders happen to be invertebrates, with a slew of their arthropod kin. In place of the backbone, spiders have rigid outer coatings.
Most invertebrates have what's called an exoskeleton, a protective, hardened outer coating. Vertebrates, on the other hand, have spinal columns and interior skeletal structure. Because their exoskeletons are on the outside and don't grow, spiders must molt, or shed their exoskeleton, a few times as they age. You may see spider "shells" in webs or other places after a molt. Other arthropods go through this process as well.
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