Which term refers to the creatures that have a defined skeletal structure?

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The term that refers to creatures with a defined skeletal structure is vertebrates. Vertebrates are a subphylum of the phylum Chordata, characterized by having a backbone or spinal column. This skeletal structure provides support and protection for the spinal cord, which is an essential part of the central nervous system. Vertebrates include a wide variety of animals such as mammals, birds, reptiles, amphibians, and fish.

In contrast, invertebrates lack this backbone and encompass a vast group of organisms, including insects, arachnids, mollusks, and many others. Prokaryotes refer to unicellular organisms that do not have a nucleus, which includes bacteria and archaea, and they do not possess specialized skeletal structures. Eukaryotes are organisms whose cells contain a nucleus, and this group includes both vertebrates and invertebrates, but not all eukaryotes have defined skeletal structures.