MORPHOLOGY OF BRAIN AND SENSE ORGANS OP LIMULUS. 55 



existence of this nerve in Arachnids. It is not easy to over- 

 estimate the importance of these facts. They prove be- 

 yond doubt that there has been no addition or sup- 

 pression of neuromeres about the mouth either of 

 Insects or of Arachnids. 



It is also obvious that in Arachnids the fore-brain^ the cheli- 

 ceral neuromere, and the principal stomodaeal nerves are respec- 

 tively homologous with the fore-brain, the antennal neuro- 

 mere^and stomodaeal nerves of Insects and Myriapods. 

 The ''pons stomatogastric" of Myriapods is homologous with 

 the frontal ganglion of Insects, and the cross-arms uniting it 

 with the ganglion of the oesophageal commissure, or what I 

 have called in Insects and Arachnids the anterior pons stomo- 

 dsei. In spiders the same organ is called by St. Remy the 

 " rostral gauglion," or " lobe rostral." The lateral rostral 

 nerves and the labral nerves throughout Myriapods, Insects, 

 and Arachnids, including Limulus, are the same, for in all 

 these cases they arise either very near to or directly from the 

 lateral stomodaeal ganglion. It is hard to understand how St. 

 Hemy, after making his careful study of the brain of Myria- 

 pods and Arachnids, could overlook, as he seems to have done, 

 these obvious homologies. It was probably due to the false 

 a priori assumption that tlie antennae are absent in Arach- 

 nids, and that their chelicerse correspond to the mandibles of 

 insects. 



We are thus able to reduce the fore- and mid-brain of 

 Myriapods, Insects, and Arachnids to the same ground plan. 

 Such a comparison has been heretofore impossible, owing to 

 the absence of the necessary embryological data. This, 

 however, has been no serious obstacle to those who find it so 

 easy to account for all discrepancies by assuming that one or 

 more neuromeres have been omitted as the necessities of the 

 comparison demanded. But there is apparently no biological 

 law more constant than that head and anterior trunk segments, 

 once formed, are never entirely omitted, or new ones intercalated 

 between them ; and a careful study shows that the head of 

 Arthropods offers no exception to this law. On the other 



