English: Drawing of the most forward thorax appendix of a late meraspis instar of Agnostus pisiformis, based on Whittington, H. B. et al. (1997). Part O, Treatise on Invertebrate Paleontology. Revised, Volume 1 – Trilobita – Introduction, Order Agnostida, Order Redlichiida, page 90, figure 79. The coxa is connected to the ventral side of the thorax. The side of the coxa facing the midline (upside in the drawing) carries spines and is called the gnatobasis. A pair of opposing gnatobases probably acted as jaws. The coxa also carries two arms: the ventral endopodite, which consist of 7 sections called podomeres (en1 to en6 and the terminal clawlike podomere) and the lateral exopodite, which consists of two podomeres (ex1 and ex2). The endopodite segments carry spines toward the midline and thumblike extensions laterally. The exopodite terminates with four spines called seta (plural setae). Total length ¼ mm.
Agnostus pisiformis has nine pairs of appendages, the antennules, and eight pairs of "legs", that differ in size and composition. The most forward pair has a small coxa, 7 podomere exopodites carrying long bristle-like setae and gill-like extentions, and no endopodite. The second pair has 5 podomere exopodite and 6 podomere endopodite. The third 3 podomere exopodite and 6 podomere endopodite. The fourth 2 podomere exopodite and 7 podomere endopodite. The fifth a single podomere exopodite and 6 podomere endopodite. Further appendages are more and more reduced in size and complexity. From Mueller, Klaus J., and Walossek, Dieter (1987). Morphology, Ontology and Life habit of Agnostus pisiformis from the Upper Cambrian of Sweden. Fossils and Strata 19:1-124.