80 WILLIAM PATTEN. 



significance of certain parts of the parietal eye in Limulus and 

 Vertebrates, that they are homologous with each other as a 

 whole is, I believe, beyond question. 



V. The Olfactory Organs. 



We have in the olfactory organ of Limulus a structure pre- 

 senting the most striking and unusual features. It is as 

 different from the other cerebral sense organs as the olfactory 

 organ of Vertebrates is from the pineal eye. The features 

 that in Limulus distinguish it from all other sense organs are 

 the very ones characteristic of the olfactory organ in Verte- 

 brates. The organ itself consists of an upright epithelium 

 containing a large number of bud-like sense organs comparable 

 with the sense organs described by Blaue in the olfactory 

 organ of fishes. The whole organ, as it exists in the adult 

 Limulus, may be compared to the unpaired olfactory organ 

 found in the Cyclostomata. It is composed of three distinct 

 parts — a pair of primitive sense organs growing forward from 

 the brain, and a subsequently formed part lying between them. 



It is probable that the dormant individuality of these 

 parts has reasserted itself in the higher Vertebrates, and 

 caused the separation of the olfactory organ into its con- 

 stituent parts, or the olfactory organ proper, and the organ of 

 Jacobson. The distribution of the nerves supports this view. 

 In Limulus there are three distinct nerves : the paired lateral 

 and hasmal ones, having their roots in one of the centres of 

 the optic ganglia, or in the optic thalami; and an unpaired, 

 median, neural nerve terminating in the cerebral hemispheres. 

 The median nerve, which in its early stages shows evidence of 

 its paired origin, arises at a late period as an outgrowth from 

 the cerebral hemisphere. It is a new formation, in histological 

 structure and origin utterly unlike any other Arthropod nerve. 

 All four of these nerves are represented in Vertebrates, for it is 

 now known that each olfactory nerve of the higher Vertebrates 

 is represented in Amphibia by two distinct nerves, which have 

 been likened to the dorsal and ventral roots of a spinal nerve. 



