Each segment of the animals body carried a pair of jointed limbs on the underside. These limbs are very rarely preserved in fossils, although x-ray techniques have been used to reveal the legs, antennae, featherlike gills and other appendages of some completely preserved trilobite fossils from the Ordovician Utica Shale of New York state, and the Devonian Hunsrück Slate of Germany.  

How the fossils of trilobites are found has a large part to do with trilobites having an articulate body encased in a hard external shell, called an exoskeleton, which was divided up into segments.  Because of these segments, some species were able to roll up as their means of defense.  In fact, trilobites are sometimes found preserved in an enrolled state.  Most of the post-Cambrian trilobites could roll themselves up into a ball when distressed.  As with different living organisms today, the trilobites shed or molted their exoskeleton periodically as the animal increased in size.  A new and larger outer shell was grown in place of the original.  The disarticulated remains of these molted exoskeletons are often found individually, or sometimes they accumulated in an area, possibly caused by the current actions of the water they inhabited.  Many have facial sutures across the head shield, where it would split so the animal could crawl out of the exoskeleton, once it had outgrown it.  Many trilobite fossils are shed exoskeletons and lack part of the head shield. A specimen containing the entire head shield, and the remainder of the specimen along with it, represents the death of that trilobite.