46 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. I42 



worms, the cephalic lobe of the arthropod embryo should contain 

 some part derived from the nonsegmental prostomium of the worms. 

 Heymons (1901) in his study of the embryo of a centipede, Scolo- 

 pendra, asserts that only the clypeal region and the labrum pertain 

 to the prostomium, and that the first three postoral segments of the 

 annelid are represented in the arthropod by an ocular segment, a 

 preantennal segment, and an antennal segment. These alleged seg- 

 ments, he says, correspond with internal nerve ganglia and with 

 mesodermal coelomic sacs. 



Small paired cavities in the preantennal mesoderm have been ob- 

 served in a number of arthropods, and there are usually mesodermal 

 sacs associated with the antennae. In several cases, also, cavities have 

 been reported in the labral mesoderm, but none has been attributed 

 to the ocular region. Weber (1952), after a review of the various 

 theories of head segmentation, gives his own conclusions as follows. 

 The arthropod head consists of a prostomial acron and six segments. 

 The acron contains the primitive brain, or archicerebrum, which in- 

 nervates the eyes. Its ventral part becomes elongate posteriorly to 

 the mouth. A preantennal segment follows the acron. Its ganglia, 

 termed the prosocerebrum, unite with the archicerebrum to form the 

 definitive protocerebrum. The preantennal coelomic sacs are often 

 suppressed or united with the second pair. Next is the antennal 

 segment, the ganglia of which become the deutocerebral component 

 of the brain. Third is the premandibular segment. Its ganglia in 

 lower Crustacea remain on the circumoesophageal connectives, but 

 in the other groups they unite with the brain as the tritocerebrum. 

 These are the ganglia of the second antennae of Crustacea, of the 

 chelicerae in the Chelicerata. The fourth, fifth, and sixth segments 

 are the mandibular, first maxillary, and second maxillary, or labial, 

 respectively. 



Weygoldt (1958) in his study of the embryonic development of 

 the amphipod Gammarus arrives at essentially the same analysis of 

 the head segmentation as does Weber. This interpretation, that the 

 head consists of a prostomium and six segments, is probably agree- 

 able to most students of the subject who contend that the embryonic 

 head lobe is a formerly segmented region of the trunk. Dahl (1956), 

 for example, says that Weber's interpretation is the one that most 

 closely agrees with his own view on the matter. 



A somewhat different scheme of head segmentation is deduced by 

 Chaudonneret (1950) from his elaborate study of the head of Ther- 

 mohia domestica. The prostomium he restricts to a very small apical 



