June, 1921] CrAMPTdN : Rvoi.UTIOX OF THE AXTHPOPODA. 



69 



investigations) we would expect that the most primitive type of 

 mandible would be more like a walking-leg than a jaw-like append- 

 age, and those arthropods in which the mandibular appendage is still 

 in tlie ■■ walking-leg stage " should therefore be taken as the starting 



Textfigures i to 6 (compare with figures in plates; are diagrammatic rcpre 

 sentations of the stages in the development of the insectan type of mandible. 

 For interpretation of lettering see list of abbreviations at end of paper. Text- 

 figure I. ^[andibular limb of trilobite. Textfigure 2. Limb of merostome. 

 Textfigure 3. Mandible of Xebalia. 



point for tracing tlie evolution of the mandibular appendage in the 

 higher forms. 



Fortunately, in such primitive artliropods as the trilobites (e.g., 

 Triarthnis hccki. which Beecher has studied with such signal success) 

 we have an excellent starting point for the study of the evolution of 

 the mouthparts of other arthropods, since in the trilobites the ap- 

 pendages which Beecher homologizes with the second antennae, 

 mandibles, first maxill.'e, and second maxilloe of Crustacea, are all 

 practically alike, and are almost exactly like the trilobite's "walking" 

 legs. In fact. I know of no other arthropods which so well illustrate 

 :he fact that the mouthi)art appendages arc merely limbs of the 

 ■ walking-leg " type adapted for holding and comminuting food. 

 Since those mouthpart-limbs of a trilobite which arc homologous with 

 ihe second antenna;, mandibles, first maxill.T and second maxillrv of 

 Crustacea, have not yet taken on the character of these appendages 

 of the Crustacea, but are still " walking "-leg (or more accurately 



