20 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 8 1 



adult head (certain crustaceans), or constitutes the basic structure to 

 which other body segments are added later to form the definitive head. 

 It is impossible, therefore, to escape the conclusion that the primary 

 embryonic head represents an early phylog-enetic stage of cephaliza- 

 tion, which was characteristic of the ancestors of all the arthropods. 

 This first arthropod head may be termed the protoccphalon (proceph- 

 alon. Patten, 1912) to distinguish it from the prostomial head of the 

 annelids, which might fittingly be designated an archie ephalon, though 

 Cram]iton (1928a) has proposed this term to denote a later formed 

 cephalic region composed of the protocephalon and the mandilnilar 

 segment. 



There has been some uncertainty as to the number of segments in- 

 volved in the protocephalon, for the segmentation of the cephalic lobes 

 is never clearly marked in the eml)ryo, and the existence of primary 

 head segments is usually indicated rather by the presence of the head 

 appendages, and by the divisions of the cephalic nerve mass, than by 

 the appearance of metamerism in the head itself. It appears most prob- 

 able, however, for reasons to be given presently, that the protocephalon 

 comprises a prostomial region and two or three primitive somatic 

 segments. The adult arthropod brain is a syncerebrum, consisting 

 always of two parts, the protocerebral and deutocerebral lobes, to 

 which in most cases are added the ganglia of a following segment, 

 which constitute then the tritocerebral brain lobes. The protocerebral 

 lobes are the most complex part of the brain, and they are probably 

 formed of elements derived from a primitive prostomial region and 

 from the ganglia of a preantennal segment. The deutocerebral lobes 

 are simple developments of the ganglia of the antennal segment. The 

 postantennal ganglia do not always enter into the composition of the 

 definitive brain, and their segment is often not a part of the proto- 

 cephalic head of the embryo, as indicated by the position of its appen- 

 dages (fig. 8 D, Put, fig. 22 B, C, Ch ; D, Pnt). 



The segmental position of the mouth has been the subject of much 

 difference of opinion. Most writers hold that the stomodeal invagina- 

 tion is situated in or Ijefore the first true head segment ; others claim 

 that it lies l)eliind the second, or even the third segment (Comstock 

 and Kochi, 1902; Holmgren, 1909, 1916; Henriksen, 1926). It was 

 long ago pointed out by Lankester (1881) and by Goodrich (1898), 

 however, that only on the assumption that all the true head segments 

 of arthropods are pastoral in position can the arthropod head seg- 

 mentation be homologized with the anterior body segmentation of the 

 annelids. Whatever part of the head is truly preoral, according to 

 this view, belongs to the prostomium. Moreover, Lankester argued, 



