54 WILLIAM PATTEN. 



An unpaired rostral nerve extends outward and back- 

 ward along the anterior ventral wall of the oesophagus into 

 the rostrum (figs. 59 — 61, r. n.). In Limulus, perhaps in 

 Scorpions, there are also two lateral rostral nerves arising 

 from the lateral stomodseal ganglion, and extending outwards 

 to the rostrum [l. r. n.). They agree in origin and distribu- 

 tion with the lateral labral nerves of Insects, with which they 

 are homologous. 



In Limulus the first post-oral cross-commissure is much 

 longer than the rest, and occupies a diflferent position on the 

 posterior under wall of the oesophagus (figs. 46, 47, c^.). It is 

 diSicult to account for this commissure if we do not regard it 

 as homologous with the posterior pons stomodsei of the 

 oesophageal ring of Insects. I cannot identify it in Scorpions 

 on account of the excessive crowding of the neuromeres about 

 the mouth, but there is no reason to doubt that it is present. 



c. Development of Lateral Stomodseal Nerves. — 

 The lateral stomodseal nerves in Scorpions and Limulus 

 arise in part from the walls of the CESophagus. Their develop- 

 ment is best studied in scorpions. They first appear in surface 

 views of Stage E ('Origin of Vertebrates from Arachnids,' 

 fig. 3, si. n.) as a pair of invaginations on the median border 

 of the cheliceral neuromere. The invagination gives rise to a 

 string of cells which at its inner end is continuous with an 

 evagination of the lateral wall of the oesophagus. By the in- 

 ward growth of the oesophagus the nerves are gradually drawn 

 out to their full extent; their inner ends are for a long time 

 continuous with the proliferating thickening of the stomo- 

 dseum, and their outer ends terminate in the lateral stomodseal 

 ganglia derived from the thick-walled invaginations on the 

 margin of the cheliceral neuromere. 



Thus, if we combine our observations on scorpions and 

 Limulus, we are able to identify in the Arachnids every cha- 

 racteristic nerve and ganglion of the stomodseal system of 

 Insects except the unpaired stomodseal nerve. It is not im- 

 possible that further observation on other forms will show the 



