264 EDWIN S. GOODRICH. 



cavity in the case of the Arachnida somewhat strengthens the 

 evidence in favour of this region representing the primitive 

 peristomial metamere. 



The Crustacea (figs. 11 and 12). 



The Crustacean head is composed of six regions. The last 

 three are obviously true metameres, post-oral in position, and 

 innervated from ganglia on the ventral nerve-cord ; they bear 

 the two pairs of maxillse and the mandibles. 



The next two regions, as we go forwards, are preoral in 

 position, carrying the two pairs of antennse, but are now 

 almost universally considered to be metameres of primitively 

 post-oral origin which have shifted in front of the mouth. 

 In the lower Crustacea (Apus, &c.) the second pair of an- 



FlG. 11. 



Pt.e. 



... -m. 



ant"^.. 



rridL. 



m 



IT 



Ventral view of an embryo of Astacus fiuviatilis (after Reichenbacli). 

 Jnl.^ and Ani.- First and second antennae. Md. Mandibles. Pr. I. 

 Procephalic lobe. 



tennse are still innervated from the oesophageal commissures 

 (Pelseneer, 13). In the higher forms the brain supplies both 

 pairs. These two segments assume the preoral position during 

 development, and their ganglia fuse with those of the most 

 anterior region to form the deuto- and trito-cerebrum of the 

 adult brain. 



There remains in front the sixth region, which bears the 

 large paired compound eyes. Ever since Milne-Edwards, in 



