RELATION OF ARTHROPOD HEAD TO ANNELID PROSTOMIUM. 263 



wliich in certain forms occupies a peristomial position in the 

 embryo, bears the same relation to tlie brain as the first seg- 



FiG. 9. 





riG. 10, 



Fig. 9. — Ventral view of aii embryo of Agelena labyrinthiea (after 



Balfour). 

 Tig. 10. — Diagrammatic plan of the anterior segments of the Araclmida. CIi. 



Clielieera. E. Eye. Pr. I. Proceplialic lobe. P. Protocerebrum. D. 



Deutocerebruni, 



Other letters as in Figs, 1 and 2. 



ment in Peripatus and insects, giving rise to the large proto- 

 cerebrum including the optic centres, Mesoblastic somites 

 are present, containing in scorpions and spiders (according to 

 Balfour, MetschnikofF, Kowalevsky and Schulgin, Laurie, and 

 Schimkewitsch) a pair of distinct coeloniic cavities. In Limu- 

 lus and some scorpions (according to Kiugsley, Kishinouie, 

 and Brauer) the ccfilom of the procephalic region is formed by 

 the forward extension of the cavities of the cheliceral segment. 

 It is unnecessary to repeat the arguments concerning the 

 homology of the procephalic lobes already used in the case of 

 the Hexapoda ; one may add that the presence of a coelomic 



