RELATION OF ARTHllOPOD HEAD TO ANNELID PiiOSTOMIUM. 261 



lobeSj from which region are developed the auterior segment 

 of the braiuj the optic ganglia, and the eyes. Three views 

 may be maintained with regard to the homology of the proce- 

 phalic lobes : (1) they represent the prostomiura of Annelids; 

 (2) they are merely the specialised auterior region of the 

 antennary segment, due to its secondary subdivision ; (3) they 

 represent a true metamere, and the first. 



In answer to the first suggestion, it may be said that they 

 would be a strangely large rudiment (anlage) for a prosto- 

 mium;^ that they are differentiated before the posterior seg- 

 ments; that, although in the Insecta no special coelomic 

 cavities have been found as yet to develop in this region, they 

 are known to occur in the similar procephalic lobes of some 

 Myriapods and Arachnids; that in their development they 

 strikingly resemble in general shape, position, and in their 

 markedly bilobed character the first segment of Peripatus 

 (antennary) ; and finally, that they give rise to the same and 

 largest segment of the brain, which includes the optic 

 centres. 



The same arguments may be used to refute the second view, 

 though perhaps not quite so convincingly. On the other 

 hand, it must be remembered that embryologists are all agreed 

 in considering the antennary segment of insects as complete 

 in itself, and therefore as not including the procephalic lobes. 



The third view, that the lobes represent the first metamere, 

 remains as the most probable. It has recently been held, if I 

 understand him rightly, by Heymons (3). 



Viallanes^ has shown, by his very careful researches on the 

 structure of the adult brain (17), that it consists in insects of 

 three segments. This conclusion is thoroughly supported by 

 embryological evidence. The first or protocerebrum, includ- 



' The procephalic lobes are also distinctly paired. The rudiment of an 

 Annelid prostomium is unpaired (except Rhynchelmis, Vejdovsky). 



' Although Viallanes speaks of these segments as belonging to three 

 " zoonites," he draws a distinction between the first as preoral, and the second 

 and third as originally post-oral. Such a view is difficult to reconcile with what 

 we know of Peripatus. 



