82 WILLIAM PATTEN. 



the optic ganglion of LimuluSj which for other reasons may 

 be regarded as homologous with the thalamencephalon, also 

 contains four nuclei, and from one of them (fig. 49) arises the 

 lateral olfactory nerve. 



But we have not yet reached the end of the similarity 

 between the olfactory organs of Limulus and Vertebrates. In 

 Vertebrates the olfactory organs are supposed to belong to a 

 system of supra-branchial sense organs, from which arise the 

 scattered sense organs of the head and lateral line. This supposi- 

 tion is based on the similarity between the method of develop- 

 ment of the organs in both cases, and by the fact that the olfac- 

 tory organ in many fishes is at first composed of a collection of 

 sense buds similar to those in, or originating from the snpra- 

 branchial sense organs. Now, although I do not attribute much 

 weight to these arguments, it is important to observe that we 

 have in Limulus the same relation between the olfactory and 

 other sense organs that we have in Vertebrates. We have, for 

 instance, in the thorax of Limulus a series of sense organs and 

 ganglia supplied by a purely sensory nerve, which in position 

 and connection is much like the supra-branchial nerve of Ver- 

 tebrates. Before I had paid much attention to these organs 

 in Limulus I was led, from a knowledge of these relations in 

 scorpions, to regard the epidermic thickening that gave rise to 

 the '' coxal ganglion" of Scorpions as homologous with a 

 supra-branchial sense organ of Vertebrates. My recent ob- 

 servations on Limulus confirm this comparison, and broaden 

 its significance in a most unexpected manner. In scorpions 

 the great ganglionic thickening on the median edge of the 

 coxal joint is a transitory structure, and there is no perma- 

 nent or definite sense organ found tliere.^ In Limulus, how- 



1 It seems to me now very probable that the mandibles on the walking 

 appendages of Limulus and the coxal spur of Scorpions, with its rudimentary 

 sense organ, represent much-shortened endopodites of the thoracic appendages. 

 In Scorpions the sense organ comes in about the same position as the rudimentary 

 appendages described by Jaworowski ('Zool. Anz.,' xiv Jr., 363) in Trochosa 

 singoriensis, and regarded by him — rightly, I believe — as the endopodites 

 of the thoracic appendages. But he has not, in my judgment, produced satis- 

 factory evidence that the structure described by him as corresponding to the 



