122 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL.97 



the head of Nebalia and of such entomostracan forms as Apus, 

 Daphnia, and others, though in form it often has a striking resem- 

 blance to the head of a hexapod mandibulate. However, in both the 

 amphipods and the isopods the head usually includes also the first 

 maxilliped somite and its appendages (fig. 50 J, iMxp), and may in 

 addition bear the second maxillipeds. The mandible acquires a secon- 

 dary anterior articulation with the cranium (I, J, K, c), by which 

 its action is limited to a hinge movement on a horizontal axis between 

 its two articular points (K, a, c). The same mandibular mechanism 

 has been independently developed in the decapod Crustacea and in the 

 pterygote Hexapoda. While the head structure of the Amphipoda 

 and Isopoda sets these groups apart from other Malacostraca, it does 

 not necessarily relate them to any other group. 



The final type of head developed in the Arthropoda is that char- 

 acteristic of the myriapods and hexapods, and must have evolved in 

 the common ancestors of these groups represented in the post- 

 crustacean, protomyriapodan section of the main arthropod stem 

 (fig. 54). The head of all the myriapod and hexapod groups is a 

 highly standardized structure, composed of the protocephalon and 

 the three gnathal somites, so closely united that little evidence of the 

 original segmentation remains, except in the presence of the appen- 

 dages (fig. 53 A), and even here the evidence is obscured by the loss 

 of the second antennae. In early ontogenetic stages, however, the 

 gnathal somites are entirely distinct from a large anterior cephalic 

 lobe that usually includes the second antennal somite, which may bear 

 vestiges of its former appendages. The Protomyriapoda must have 

 had compound eyes, since eyes of the compound type recur finally 

 in the Hexapoda; they likewise must have carried paragnathal lobes 

 of the head from the Crustacea to the Hexapoda, though these organs 

 have disappeared in the modern myriapodous forms. The maxillary 

 appendages probably were no more specialized in the protomyriapods 

 than in modern Chilopoda (fig. 53 A, C). The mandibles lost the 

 telopodites, but they developed a special feature of which no sug- 

 gestion is to be found in the Crustacea, namely, a mobile gnathal lobe, 

 the lacinia, movable by muscles arising in the mandibular base and 

 on the walls of the cranium. The mandibular lacinia is retained as 

 a movable lobe in modern Symphyla (fig. 52 E, Lc) and Diplopoda; 

 in the Chilopoda it is not separated from the stipital region of the 

 mandible (fig. 53 E, F), though it is provided with strong stipital 

 and cranial muscles (F, 75, 10) ; in the Pauropoda and Hexapoda 

 (fig. 52 F) apparently it has united with the stipes {St), producing 

 a solid jaw of the crustacean type, and its muscles have disappeared. 



