THE PROCEPHALON. 



13 



Met.C 



v.mt. 



Br.C 



brain with its olfactory lobes, hemispheres, and its appropriate sense organs are 

 derived. (Figs. 14-21.) The structure of the procephalic lobes, their main 

 divisions, and the relations of the three sets of primary sense organs to them, 

 are practically identical throughout the arthropod series. In the higher arachnids, 

 their structure and mode of development, and that of their associated sense organs, 

 resembles that of the vertebrates. 



In Insects (Acilius), the procephalic lobes consist of three segments, each one 

 containing a neuromere, an optic ganglion, a segment of the marginal plate, and 

 two pairs of segmental sense organs, or 

 ocelli. (Fig. 14.) Three infoldings occur 

 on the margins of the lobes, between the 

 optic plate and the optic ganglia, iv^ ', 

 but they soon close without involving the 

 marginal sense organs, and without form- 

 ing a common cerebral vesicle. 



In the Arachnids (scorpion), the lobes 

 are at first similar to those of Acilius; later 

 they are depressed, and a thin marginal 

 fold, or neural crest, advances over them, 

 converting the entire forebrain into a hol- 

 low vesicle that for a long time opens to 

 the exterior through an anterior neuropore. 

 (Figs. 15, 16, 18, 46, 47, an. p. and eph.) 



Sense Organs. — Meantime the anterior 

 pair of marginal sense organs move forward 

 and unite in the median line to form the 

 anlage of the olfactory organ (Limulus). 

 (Figs. 38, 39, 141, 142, 153, ol.o.) The two 

 pairs of sense organs on the second seg- 

 ment (ocellar placodes, parietal eyes) are 

 ingulfed in the palial overgrowth and carried 

 to the middle of the roof of the forebrain 

 vesicle. Here a tubular outgrowth is for- 

 med, on the end of which the ocellar placodes are located, after the manner 

 of a typical parietal eye. (Figs. 46, 47, 57, 141, 142, pa.e.) In the arach- 

 nids, the sense organs of the third segment (lateral eye) lie for a time on the 

 outer margin of the neural crests, but later they move away from them, 

 so they are not ingulfed in the pahal overgrowth. (Fig. 16, A.l. e.) The lateral 

 eyes of insects, crustaceans, and arachnids appear to belong to the fourth neuro- 

 mere (antennal or cheliceral), that is, to the first metamere of the next division 

 of the head. 



Olfactory Lobes, Hemispheres and Optic Ganglia. — During the formation of 

 the palial overgrowth, the first forebrain segment becomes deeply infolded to 



Fig. 6. — Mesonacis (Olenellus) vermontana 

 (Hall). Lower Cambrian, showing body regions, 

 and groups of like metameres, or tagmata. 



