EELATION OF ARTHROPOD HEAD TO ANNELID PROSTOMIUM. 265 



1834<, put forth the view that the stalked eyes of Crustacea 

 represented a pair of metameric appendages, observers have 

 been divided into two camps; some supporting this theory 

 (Huxley, Reichenbach, Nussbaum, &c.), others opposing it. 

 The latter maintain that the eyes, whether stalked or sessile, 

 are merely prostomial sense-organs. 



In the embryo we find, in this region, two large procephalic 

 lobes from which are developed the eyes and optic centres and 

 the anterior segment of the brain, corresponding to the pro- 



FiG. 12. 



ant 



Diagrammatic plan of the anterior segments of the Crustacea. Ani.^ and 

 Ant? First and second antennae. E. Eye. F. P. Frontal process. 

 Md. Mandible. Mx} and Mx?' First and second maxillae. Pr. I. Pro- 

 cephalic lobe. P. Protocerebrum. D. Deutocerebrum. T. Tritocere- 

 brum. Other letters as in Figs. 1 and 2. 



tocerebrum of other Arthropods (Viallanes). Unfortunately, 

 no definite evidence has been obtained with regard to the 

 metameric nature of this region from the study of the meso- 

 blast, since no distinct somites or coelomic cavities have been 

 traced with certainty in the anterior segments of most Crus- 

 tacea. It is evident, however, that although the procephalic 

 lobes may represent the first metamere, the stalked eyes need 

 not necessarily be its true metameric appendages. Neverthe- 



