These appendages can become quite specialized in various groups. The mouth and appendages of the living trilobite were located on the ventral side, and were protected by the dorsally located sclerocized skeleton. The non-sclerocitized ventral elements are only rarely preserved. The cephalon may or may not contain compound eyes. The thorax is composed of a number of simple segments, which in living trilobites supported legs.
Thoracic segments are the most common trilobite skeletal elements preserved in the fossil record.
The biology of the Arthropoda. Elsevier Publishing Company, New York. Eldridge, N. 1977. Trilobites and evolutionary patterns. Levi-Setti, R. 1975.