56 WILLIAM PATTEN. 



hand, to assume that there have been such changes raises an 

 insurmountable barrier to any satisfactory comparison between 

 the brain of Arachnids and other Arthropods. There is no 

 reason to doubt that the brain of Crustacea will fall in line 

 with the above comparisons. 



D. Comparison with Annelids. — Still another advantage 

 to be derived from my interpretation is that it enables us to 

 compare the cephalic lobes and stomodseal nerves of Arthropods 

 with those of Annelids. According to Lang, the paired 

 stomodseal nerves of Annelids arise from the oesophageal 

 collar, that is near the point of union of the ventral 

 cord with the brain, just as they do in Arthropods. 

 According to Kleinenberg, they originate in Lopadorhynchus 

 from the lateral margin of the first post-oral neuromere. The 

 origin of this nerve, therefore, in Annelids and in Arthropods 

 marks the point of union of the neuromeres of the head and 

 trunk. Moreover, since the stomodseum nerves arise from the 

 first post-oral neuromere of Annelids and from the first 

 neuromere of the ventral cord of Arthropods, i. e. from the 

 antennal neuromere of Insects and Myriapods, the cheliceral 

 of Arachnids, and the first antennal of Crustacea, these 

 neuromeres must be homologous. 



E. Nature of Stomodseal Nerves. — The remarkable 

 mode of development of the stomodseal nerves from the walls 

 of the stomodseum, their constant presence, and their 

 voluminous size in the early embryonic stages emphasises 

 their importance, and indicates that they belong to a system 

 quite apart from that of the fore-brain and ventral cord. If 

 so, what is their significance? The stomodseum undoubtedly 

 represents the invaginated ectoderm formerly surrounding a 

 primitive mouth leading directly into the mesenteron. That 

 primitive mouth now lies at the junction of the mesenteron 

 with the stomodsEum. We have only to turn the stomodseum 

 back to its primitive condition as ectoderm surrounding the 

 mouth, in order to obtain a clearer idea of the original position 



