22 



SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 8l 



stage lie on a line posterior to the mouth. Later, this segment loses 

 its identity, and it can not be traced in the composition of the adult 

 head. The second metamere is the antennal segment, bearing the 

 antennae of chilopods (fig. 14 A, Ant) and insects (fig. 13 A, Ant), 

 or the corresponding first antennae (antennules) of Crustacea (fig. 

 22 A, I Ant) . The third metamere is the so-called intercalary segment, 

 marked by a pair of coelomic sacs and corresponding ganglia in 

 Scolopcndrd (fig. 14 A, IIIGng), bearing the highly developed second 

 antennae of Crustacea (fig. 22 A, 2Ant) , the chelicerae of Arachnida 

 (fig. 22, B, C, C/i), or the rudimentary post-antennal appendages of 

 insects (fig. 22 'D,Pnt). The fourth, fifth, and sixth metameres of 

 the definitive chilopod head are the segments of the gnathal appendages 

 (fig. 14 A, Md, iMx, 2Mx), which have united with the proto- 

 cephalon. 



The adult brain of Scolopendra, Heymons finds, is a composite of 

 preoral and postoral ganglionic elements. The preoral parts are de- 

 rived from the ectoderm of the prostomial region, the postoral parts 

 are the paired ganglia of the first three head metameres. The pro- 

 stomial elements include a median archicerebral rudiment that becomes 

 the anterior part of the supraoesophageal commissure, and paired 

 lateral rudiments, which form the dorsal cortical plate, the frontal 

 lobes, and the optic lobes of the definitive brain. The ganglia of the 

 first metamere, or preantennal segment, are a pair of small nerve 

 masses which unite with the prostomial rudiments to form the proto- 

 cerebral lobes of the adult brain. The ganglia of the antennal segment 

 constitute the deutocerebrum ; those of the postantennal segment be- 

 come the tritocerebral lobes. The definitive location of the preantennal 

 and antennal ganglia anterior to the mouth is a secondary one, and 

 their union before or above the stomodeum, Heymons explains, comes 

 about ontogenetically through the late development of the transverse 

 commissures, which are not formed until the respective ganglia ha^'e 

 acquired a preoral position. Wheeler (1893) had sviggested that " the 

 arthropod protocerebrum probably represents the annelid supraoesoph- 

 ageal ganglion, while the deuto- and tritocerebral segments, orig- 

 inally postoral, have moved forward to join the primitive brain." 

 This essentially is also Heymon's earlier view ( 1895) , but the existence 

 of a separate pair of preantennal segmental ganglia was not suspected 

 at that tim'e. 



For many years Heymons' observations on the development of the 

 head of Scolopendra have remained unverified. It is, therefore, of 

 particular interest to find essentially the same structure now described 

 for an insect. Wiesmann (1926), studying the development of a 

 phasmid. Ca ran si us morosns, reports that the head is composed of 



