j^ OUTLINE OF THE ARACHNID THEORY. 



form the olfactory lobes. The cerebral hemispheres arise from the median part 

 of the second segment, the optic ganglia of the parietal eye (ganglion habenula), 

 from the lateral margin of the second segment, and the ganglion of the lateral 

 eyes (tectum opticum), from the lateral lobes of the third or fourth segment. 



(Figs. 15,46, 47> oil) 



The Rostrum (labrum) in insects arises as a pair of small cephalic appen- 

 dages, on the very anterior median margin of the cephalic lobes. (Fig. 14.) In 

 the arachnids it forms an unpaired, immovable process, which in the later stages 

 lies on the anterior margin of the mouth. (Figs. 15, 17, 18, 43^ 47-) It differs 

 from all other arthropod appendages, in that it receives its nerves from ganglia 



FIG. 7. Primitive Crustacea seen from the neural surface, showing various arrangements of the procephalic sense 

 organs. A, Sida; B, Limnadia larva; C, Branchipus larva. 



situated on the median side of each nerve cord, that is, from the stomodaeal ganglia 

 and commissure, which are situated near the fourth, or first post-oral, segment. 

 (Figs. 38 and 39, st.g.) 



External Boundaries of the Procephalon in the Adult. The margins of the 

 ectodermic area covering the outer surface of the forebrain, after the palial over- 

 growth is formed, mark the boundaries of the primitive head. The latter becomes 

 greatly distorted by the forebrain flexure, which carries the anterior part of the 

 forebrain round the end of the egg onto the future haemal surface, while the pos- 

 terior part is drawn a long way backward by the caudad migration of the mouth 

 and rostrum. (Figs. 3, 17, 43, 44, 46.) It thus happens that the neural surface 

 of the procephalon is the only one that is actually developed. The haemal surface 

 is not formed from procephalic tissue, but by the extension of the lateral and 

 anterior margins of the procephalic lobes around the anterior end of the ovum, 



