Arthropod phylogeny—a modern synthesis*

Factual evidence provided by the fields of functional morphology and of the newer functional studies of embryology using the concept of fate maps indicates that the Arthropoda are probably polyphyletic and that arthropodization has occurred at least three times, forming the phyla Crustacea, Chelicerata and Uniramia (Onychophora, Myriapoda, Hexapoda). When the latter left the sea they probably possessed a soft body and lobopodial limbs and showed incipient cephalization. Only the muscular haemocoelic mechanism of movement of the lobopodium, but not that of a parapodium, could have given rise to the jointed uniramian limbs. The three subphyla of the Uniramia are differentiated by their jaw mechanisms, associated head structure and limb-bases. The habits associated with the divergence of the four myriapodan classes account for their morphological distinctions. The five hexapod classes differ from one another in leg-base mechanisms and have probably been evolved independently from five groups of multilegged soft bodied ancestors with a head type contrasting with that of the Myriapoda. Reference is made to the great differences between arachnid adaptations of limbs, gaits, etc., to land life compared with those of the Uniramia. No new evidence is given concerning the status of the Trilobita.