82 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. Si 



in structure to serve as organs accessory to the mandibles, but they 

 have not attained the highly specialized form of the corresponding 

 appendages of insects. 



We may conclude, therefore, that in the common ancestor of the 

 several groups of modern eugnathate arthropods, the mandible alone 

 had attained a gnathal function, and that in form and structure it re- 

 sembled the maxilla of a present day insect, though perhaps lacking 

 a galea, or outer endite lobe of the base. The two maxillae at this 

 period were more or less modified to serve as organs accessory to the 

 mandibles. 



When the modem groups of arthropods were differentiated, the 

 mandible, in the Diplopoda and Chilopoda, retained the movable 

 lacinia, but lost the palpus ; in the Crustacea and Hexapoda, the lacinia 

 fused with the base of the appendage to form a solid jaw, while the 

 palpus was preserved by the crustaceans, and lost by the insects. The 

 two maxillary appendages retained the leg-like form in the Chilopoda ; 

 in the Diplopoda they became highly specialized in a manner peculiar 

 to the diplopod group ; in the Crustacea and Hexapoda they were 

 modified for an accessory gnathal function, but in the insects they ac- 

 quired a form almost identical with that of a primitive mandible. 

 Finally, in the insects, the second maxillae united basally to form the 

 labium. While the insect maxilla appears to be a highly specialized 

 appendage, it will be shown that its basic structure is not far removed 

 from that of a thoracic leg. 



While the status of the gnathal appendages relative to one another 

 in the various groups of the eugnathate arthropods seems fairly clear, 

 it is a more difficult matter to homologize their parts with the segments 

 of the ambulatory appendages. The structure of the first and second 

 maxillae of a chilopod, or of the first maxilla of an insect, suggests 

 that the gnathal appendages have been derived from an appendage 

 of the ambulatory type — the insect maxilla is certainly more like the 

 leg of an insect, a chilopod, or a decapod than it is like one of the 

 body appendages of Apits (fig. 35 C), or of any other of the lower 

 crustaceans in which the appendages are used for swimming. This 

 condition suggests, therefore, that the ambulatory leg more nearly 

 represents the primitive type of arthropod limb than does an appen- 

 dage, such as that of Apiis, clearly modified for purposes of purely 

 aquatic locomotion. If we consider, furthermore, that the appendages 

 of the chelicerate arthropods (Xiphosura and Arachnida) are also of 

 the ambulatory type, the evidence becomes all the more convincing 

 that the primitive arthropod limb was a walking leg and not a swim- 

 ming organ. If this deduction is acceptable, we must conclude that 



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