NO. 3 INSECT HEAD SNODGRASS 21 



the arthropod brain must contain a median anterior rudiment derived 

 from the prostomial ganglionic mass, or archicerebrum, in addition to 

 the ganglia of the component segments. " In the Chaetopoda," Lan- 

 kester says, " the prae-oesophageal ganglion appears always to remain 

 a pure archicerebrum. But in the Crustacea (and possil)ly all other 

 Arthropoda * * * ) the prae-oesophageal ganglion is a syn-cerebrum 

 consisting of the archicerebrum and of the ganglion masses appropri- 

 ate to the first and second pair of appendages which were originally 

 postoral, but which have assumed a praeoral position whilst carrying 

 their ganglionic masses up to the archicerebrum to fuse with it." 



According to Lankester's view, then, the arthropod head should 

 comprise a prostomial region and several postoral segments, and the 

 brain correspondingly should include the prostomial archicerebrum 



Clp ^ Prnt 



Lm 



IIIGr 



Ant 





~Md 



A ^-^ B 



iMx 



Fu;. 14. — Young embryos of a chilopod and an insect showing rudiments of 

 preantennal appendages. 



A, anterior end of embryo of Scolopcmira (from Heymons, 1901). B, same 

 of a phasmid, Carausiiis niorostis (from W'iesmann, 192(1). 



Ant. antenna; Clp, clypeus ; Hphy, hypopharynx ; IIIGng, tritocerebral gan- 

 glion ; Lm, labrum ; Md, mandible ; iMx, first maxilla ; 2Mx, second maxilla ; 

 Pnit, preantenna. 



and the paired postoral ganglia of the first two segments, with the 

 ganglia of the third segment added in most cases. This idea, expressed 

 theoretically by Lankester and by Goodrich, has been given substantial 

 support by Heymons in his study of the development of Scolopendra, 

 and more recently by Wiesmann from a study of the embryo of 

 Carausius. 



The head of Scolopendra, Heymons (1901) says, is formed during 

 embryonic development by the union of an unsegmented preoral 

 region and six postoral segments. The preoral part Heymons calls 

 the " acron," taking this term from Janet ( 1899) in a slightly altered 

 sense ; it is the primary " Kopfstiick," which clearly is the pro- 

 stomium. The first true metamere, or postoral segment, is marked by 

 a pair of small coelomic sacs in the mesoderm, and bears a pair of 

 evanescent preantennal appendages (fig. 14, Prnt), which at an early 



