112 WHEELER. [Vol. VIII. 



tritocerebral segment, it cannot be homologizecl with the 

 Annelid Scheitelplatte, a structure which is not segmented. 



A view midway between Will's and Patten's probably accords 

 best with the facts at our disposal. The Arthropod proto- 

 cerebrum probably represents the Annelid supraoesophageal 

 ganglion, while the deuto- and tritocerebral segments, 

 originally postoral, have moved forward to join the primitive 

 brain. This is essentially Lankester's view ('8l), according to 

 which in Arthropods *'the praeoesophageal ganglion is a syn- 

 cerebrum consisting of the archicerebrum and of the ganglion 

 masses appropriate to the first and second pair of appendages 

 which were originally postoral, but have assumed a praeoral 

 position whilst carrying their ganglion-masses up to the archi- 

 cerebrum to fuse with it." 



In comparing the Arthropod with the Annelid brain much 

 stress has been laid on the fact so clearly brought out by 

 Kleinenberg ('86) — that the Annelid supraoesophageal ganglion 

 originates independently of the ventral nerve-cord. Several 

 investigators — Balfour ('BO), Schimkewitch ('87), Will ('88) and 

 others — have fancied that they could detect a similar onto- 

 genetic discontinuity of the brain and nerve-cord in Arthropods. 

 But more recent observations all tend to prove that there is 

 a direct continuity of the central nervous system from the time 

 when the ganglia first make their appearance. So far as the 

 insects are concerned I may note that Will's conclusions were 

 based on the defective surface observation of a form [Aphis) ill 

 adapted to the study of the central nervous system.^ 



Even granting that the Annelid brain arises independently of 

 the nerve-cord — and this is not yet settled — at least so far as 

 the Oligochaeta are concerned (see E. B. Wilson, '89) — Lankes- 

 ter's view of the Arthropod brain is in no way invalidated. The 

 line of separation corresponding to the Annelid prototroch must 

 fall in front of the deutocerebral segment, since it has been 

 shown that this segment in some insects contains a pair of well- 



^ Little value can be attached to Cholodkowsky's assertion that in Blatta the 

 supraoesophageal ganglion originates independently of the nerve-cord, since he has 

 failed to see the deuto- and tritocerebral segments which are quite as well 

 developed in Blatta as in other Orthoptera. 



