COMPARISON WITH VERTEBRATES. 1 69 



phyllopods, and that in both classes it may undergo varying degrees of fusion, or 

 of unequal development of its constituent parts. 



2. Number of Nerves. — In the arthropods, the olfactory organ always 

 shows traces of two pairs of nerves, even when the organ itself is practically 

 unpaired. I pointed out in 1893 that the two pairs of olfactory nerves, 

 then known in but a few vertebrates, were comparable with the two pairs in 

 Limulus, but not with any other cranial nerves known elsewhere, either in verte- 

 brates or invertebrates; I stated that: "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. But if 

 this were so they would differ from all other spinal nerves in that both dorsal and 

 ventral branches supply sense organs. Moreover, on any supposition they are 

 entirely different from those belonging to the other sense organs of the forebrain, 

 such as the lateral and parietal eye, and the auditory organ. This condition is 

 quite inexplicable on any theory founded on vertebrate anatomy. But this very 

 thing occurs in the olfactory organ of Limulus, although the meaning of it cannot 

 be explained there any more than in vertebrates." 



It is interesting to recall the statements made at that time, since they have 

 been in some respects so fully confirmed by the subsequent discovery of two pairs 

 of olfactory nerves by Pinkus 1894, in Protopterus; by Allis 1897, in Amia; by 

 Locy 1899, in the elasmobranchs; and by Zewertzoff 1902, in the embryos of Cer- 

 adotus. If the condition in Limulus had received more serious consideration, it is 

 very possible that the little "foot note" to the ancestral history of the vertebrate 

 brain, which according to Locy, is furnished by the development of the nervus 

 terminalis, might have expanded into a chapter. 



3. Structure and Termination of the Nerves. — Arthropods. Both pairs 

 of nerves, while supplying the same organs, are widely different in their histo- 

 logical characters, and in their central termination. Both pairs are ganglionated. 

 The lateral nerves contain very coarse nerve fibers with distinct sheaths, and 

 scattered clusters of gigantic ganglion cells; they terminate in the base of the brain, 

 near the roots of the optic tracts. The median nerve contains fine sheathless 

 fibers, dense masses of neuropile and small ganglion cells; it has its roots in 

 the olfactory lobes and in the hemispheres. 



The olfactory nerves are "sui generis" and are only remotely comparable 

 with any other cranial nerves, such as the optic nerves, the segmental gustatory 

 nerves, or with the components of less specialized peripheral nerves. 



Verebrates. According to Locy, both pairs of olfactory ner\'es are ganglion- 

 ated, and although closely associated in their peripheral termination, have sepa- 

 rate central origins, hence they are considered to be separate ner\es, not as 

 separate parts of one nerve. 



Whsit Pinkus says of the nervus terminalis, viz., "Eine kolbige Anschwellung 

 dieses Nerven, welche durch die Einlagerung grosskerniger, von alien anderen 

 nervosen Zellen des Protopterus anscheinend verschiedenen Zellen bedingt ist. 



