The adult chilopod lacks the antennules, and all of the other appendages, with the exception of the maxillulÆ, are uniramous. The walking legs are similar to the endopodites of trilobites, and usually have six or seven segments. The appendages are therefore such as could be derived by modification of those of trilobites by the almost complete loss of the exopodites and shortening of the endopodites of the head. The position of the post-oral appendages, the posterior ones outside those closest the mouth, is perhaps foreshadowed in the arrangement of those of Triarthrus.

The Chilopoda differ from the Hexapoda in developing the antennÆ instead of the antennules as tactile organs, but this can not be used with any great effect as an argument that the latter did not arise from the ancestors of the former, since it is entirely possible that in early PalÆozoic times the pre-Chilopoda possessed two pairs of antennÆ. The first pair are still recognizable in the embryo of certain species.

The oldest chilopods are species described by Scudder (Mem. Boston Soc. Nat. Hist., vol. 4, 1890, p. 417, pl. 38) from the Pennsylvanian at Mazon Creek, Grundy County, Illinois. Only one of these, Latzelia primordialis Scudder (pl. 38 fig. 3), is at all well preserved. This little animal, less than an inch long, had a depressed body, with a median carina, exceedingly long slender legs, and about nineteen segments. The head is very nearly obliterated.