INSECTA AND CHILOPODA. 123 



of the test to be easily transformed into movable organs, but they are structurally too unlike 

 the veined wings of insects to make the suggestion of this derivation even worthy of con- 

 sideration. 



Tothill (1916) has recently reinvestigated the possible connection between insects, chi- 

 lopods, and trilobites, and, from the early appearance of the spiracles in the young, came to 

 the conclusion that the insects were derived from terrestrial animals. He suggested that they 

 may have come through the chilopods from the trilobites. The hypothetical ancestor of the 

 insects, as restored by Tothill from the evidence of embryology and comparative anatomy, 

 is an animal more easily derived from the Chilopoda than from the Trilobita. Five pairs of 

 appendages are present on the head, and the trunk is made up of fourteen similar segments, 

 each with a pair of walking limbs and a pair of spiracles. 



Only the maxilla; and maxilluke are represented as biramous. If the ancestor of the 

 Insecta was, as seems possible, tracheate, this fact alone would rule out the trilobites. 

 Among tracheates, the Chilopoda are certainly more closely allied to the Insecta than are 

 any other wingless forms. If the ancestors of the insects were not actually chilopods, they 

 may have been chilopod-like, and there can be little doubt that both groups trace to the 

 same stock. 



As to the ancestry of the Chilopoda, it is probable that they had the same origin as 

 the other Arthropoda. Tothill has pointed out that in the embryo of some chilopods there 

 are rudiments of two pairs of antenna? and that the two pairs of maxillre and the maxilli- 

 peds are biramous. This would point rather to the Haplopoda than directly to the trilobites 

 as possible ancestors, and may explain why the former vanish so suddenly from the geological 

 record after their brief appearance in the Middle Cambrian. They may have gone on to 

 the land. 



There seem to be no insuperable obstacles to prevent the derivation, indirectly, of the 

 insects from some trilobite with numerous free segments, and small pygidium. The anten- 

 nules and pleural lobes must be lost, the antennas and trunk limbs modified by loss of exopo- 

 dites. Wings and tracheae must be acquired. 



Handlirsch places the date of origin of the Insecta rather late, just at the end of the 

 Devonian and during the "Carboniferous." By that time most families of trilobites had 

 died out, so that the possibilities of origin of new stocks were much diminished. If the 

 haplopod-chilopod-insect line is a better approximation to the truth, then the divergence began 

 in the Cambrian. 



CHILOPODA. 



The adult chilopod lacks the antennules, and all of the other appendages, with- the ex- 

 ception of the maxilluke, 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 postoral ap- 

 pendages, 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 antenna; 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 Palasozoic times the pre-Chilopoda possessed two pairs of antennae. The first 

 pair are still recognizable in the embryo of certain species. 



