This is the largest phylum of animals today, and was similarly prominent during the Paleozoic. Arthropods have non-cellular exoskeletons made up of cuticle. In marine arthropods the exoskeletons are often biomineralized with calcium carbonate, giving them extra strength and rigidity. Arthropods have segmented bodies and paired jointed legs and antennae.
Contemporary arthropods include insects, crustaceans, spiders, and millipedes. All of these types of arthropods lived by the latter part of the Paleozoic, and, in the case of crustaceans, from nearly the very beginning of the Paleozoic. But the Paleozoic also had types of arthropods that are long extinct, including trilobites and euryptids.