26 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 8l 



long, segmented body, at the anterior end of which was a specialized 

 cephalic region, differing from the annelid head in that it comprised 

 both the prostomium and the first two or three primitive body seg- 

 ments. In this early arthropod head, or protocephalon, the prostomium 

 was still an important element ; it perhaps carried the ocular organs, 

 though tentacles were probably lacking, and it was extended dorsally 

 on the facial aspect of the head between the bases of the antennae ; 

 on its ventral part, just before the mouth, there was a median lobe, 

 the labrum. The first true head segment was much reduced, and its 

 appendages were vestigial, or absent, unless they are represented in the 

 eye-stalks of modern Crustacea. The second head segment bore the 

 antennae, simple, jointed appendages, which acquired a preoral position 

 on the sides or front of the head by a secondary forward migration 

 of their bases. These two segments and the prostomium became in- 

 timately fused, and in the ontogenetic development of present-day 

 arthropods they appear as a unified, bilobed cephalic enlargement of 

 the young embryo (figs. 8, 13, 16 A, B, C, 22 C, D, Pre). The brain 

 at this stage was a syncerebrum, consisting of the archicerebrum and 

 optic lobes fused with the ganglia of the preantennal and antennal 

 segments, the two lateral nerve masses being united above the stomo- 

 deum (fig. 15 B). The third postoral segment was probably more 

 or less closely associated with the second, but, judging from embryonic 

 evidence (fig. 8D), it did not at first form an integral part of the 

 protocephalon. Its ganglia (later the tritocerebral lobes of the brain) 

 at this stage constituted the first ganglia of the ventral nerve cord. 



There can be no question that the arthropods are to be divided into 

 two principal groups, one represented by the modern mandibulate 

 forms, the other by those in which the appendages of the fourth seg- 

 ment retained the more generalized structure of the pedipalps of 

 modern arachnids and xiphosurans. The separation of the two groups 

 must have taken place in the protocephalon stage, for, as will later 

 be shown, the unity of structure in the mandibles of all the mandibu- 

 late forms is such as to leave no doubt that the mandible is a common 

 inheritance from a primitive mandibulate ancestor. But, before the 

 definitive gnathal segments were added to the head, it would seem 

 that the postantennal, or tritocerebral, appendages must have as- 

 sumed the principal gnathal function by means of basal endites that 

 served as masticatory lobes. In the xiphosurans and arachnids, these 

 appendages have become the chelicerae, if modern embryology is 

 rightly interpreted ; in the crustaceans they lost their gnathal function 

 and were developed into the second antennae ; in the land-inhabiting 



