4 WILLIAM PATTEN. 



of the endoparietal eye ; comparison with white pigment in the eye of Pe- 

 tromyzon ; Gaskell's comparison of parietal eyes without sufficient data. 



V. Oltactoky Organ, p. 79. 

 Comparison with olfactory organ in Vertebrates ; its four nerves correspond 

 to the four olfactory nerves of Amphibians. Relation in Limulus and Ver- 

 tebrates of the olfactory nerves to the optic thalami ; comparison of the 

 olfactory organs of Vertebrates and Limulus with the supra-branchial or 

 mandibular sense organs. The function, distribution, and multiplication of the 

 mandibular sense organs of Limulus are like those of the supra-branchial 

 organs of Vertebrates. The mandibles probably represent the rudimentary 

 endopodites of the thoracic appendages. 



VL COMPAKISON OF OTHER BrAIN REGIONS OF LiMULUS "WITH THOSE 



OF Vertebrates. 

 Comparison of the fore-, mid-, hind, and accessory brains and their nerves 

 with the corresponding regions in Vertebrates, p. 83. Concluding remarks, 

 p. 85. 



Introduction. 



Some two years ago I published ashort paper in this Journal 

 calling attention to many striking resemblances between 

 Arachnids and Vertebrates. I maintained that the Verte- 

 brates are descended from a great group of Arthropods, 

 in which I included the Arachnids, Trilobites, and Mero- 

 stomata ; and that the remarkable palaeozoic fishes Pterichthys, 

 Bothriolepis, and Cephalaspis resemble merostomatous Arthro- 

 pods like Pterygotus, Eurypterus, &c. Now, the internal 

 structure of the Merostomata and Trilobites cannot differ 

 greatly from that of Limulus, judging from their resemblance 

 in external characters; therefore, although Limulus itself is 

 not in the main line of Vertebrate descent, a study of its 

 structure will best enable us to understand that of the Mero- 

 stomata and of the primitive Vertebrates. 



In my preliminary paper the lines were indicated along 

 which I had found evidence of relationship between Ver- 

 tebrates and Araclinids. It was shown — and this first drew 

 my attention to the subject — that the invaginations, which in 

 insects give rise to the optic ganglia, in scorpions and Limulus 



