124 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL.97 



The maxillary appendages in Symphyla and Hexapoda have acquired 

 two endite lobes of the stipes (lacinia and galea), but the palpi have 

 been lost in Symphyla (B). 



The last important event in the evolution of arthropod head appen- 

 dages was the union of the bases of the second maxillae to form a 

 single median organ, the so-called labium. The labium took its origin 

 in the common ancestors of the Symphyla, Diplopoda, Pauropoda, 

 and Hexapoda, which constituted the third and most prolific branch 

 of the arthropod stock (fig. 54). The primitive structure of the 

 labium is best preserved in the more generalized hexapods (fig. 

 52 D) ; in the Symphyla (C), Pauropoda, and Diplopoda (G) it has 

 lost the telopodites, or palpi, and in the diplopods it forms at least a 

 part of the complex gnathochilarium (G). 



Crampton's (1928) phylogenetic conclusions drawn from com- 

 parative studies of the arthropod head differ radically in some respects 

 from the concept of arthropod relationships here deduced from the 

 same source. Crampton believes that the first arthropods derived 

 from annelid precursors were probably prototrilobites, and that from 

 the latter were evolved in one direction the Trilobita and Chelicerata, 

 in another the Protocrustacea, which last in turn produced the higher 

 Crustacea, while finally, from the malacostracan Crustacea were 

 evolved the Myriapoda and Hexapoda. 



To the writer it would seem that if the Protarthropoda are con- 

 ceded to have been derived from wormlike ancestors, whether anne- 

 lidan or protonychophoran, by a sclerotization of the integument and 

 a jointing of the appendages, they must have taken on at once a 

 centipedelike form. According to the theory here proposed, therefore, 

 a long, unbroken line of slender polypodous arthropods has persisted 

 from the ancient protonychophoran progenitors to the modern chilo- 

 pods. Along this line have been carried the features common to all 

 the arthropods, while new characters evolved in the main line itself 

 have been distributed to subsequent lateral branches, where in some 

 cases they have persisted in their original state, in others they have 

 still further evolved, and in still others they have been lost. 



The first lateral branch from the arthropod stem was that of the 

 Prototrilobita (fig. 54), which produced the Trilobita and the Chelic- 

 erata. In this branch cephalization united the first four somites with 

 the acron to form the trilobite "head," and continued in the Chelicer- 

 ata until the "prosoma" contained six and eight somites. Meanwhile, 

 in the main protarthropod stem, cephalization produced a more simple 

 head (protocephalon) consisting of the acron and only the first somite, 

 but the appendages of the second somite were converted into a pair 



