MORPHOLOGY OP BRATN AND SENSE ORGANS OF LIMULUS. 81 



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 

 fore-brain, 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 LimuluSj although the meaning of it 

 cannot be explained any more there than in Vertebrates. 



Now in Pipa dorsigera and Epicrium glutinosum, 

 according to Wiedersheim, there are four distinct olfactory 

 nerves — a neural and haemal pair. The Saracins in their 

 Ceylon work, and more recently Burckhardt ('Z. f. w. Zool.,^ 

 1891), have further shown that in Ichthyophis and Triton the 

 hsemal pair innervate the organ of Jacobson, and the neural 

 pair the rest of the olfactory organ. Exactly the same rela- 

 tions prevail in Limulus ; the haemal pair are also lateral, and 

 they supply a distinct lateral depression in the olfactory organ. 

 The neural pair in Limulus innervates the greater part, if not 

 the whole, of the definitive olfactory organ ; they are formed 

 by an outgrowth from the cerebral hemispheres ; they contain 

 the olfactory bulbs or lobes, and they are composed of pale 

 refractive nerve-fibres with a structureless nucleated sheath 

 differing strikingly from the thick- walled apparently empty 

 nerve-tubes seen in the lateral olfactory and other nerves • 

 moreover, as they supply by far the greater part of the olfac- 

 tory buds with nerves, they may be regarded as the olfactory 

 nerves proper. They therefore agree in these important mor- 

 phological and histological features with the dorsal pair in 

 Amphibians. In the higher Vertebrates all four nerves seem 

 to have united, and the features of the median pair impressed 

 on both. 



I am not aware that the course of the roots to the olfactory 

 nerves is accurately known in the lower Vertebrates; but in the 

 Mammalia there are four large medullary nuclei in the thala- 

 mencephalon, in one of which terminate the lateral roots to the 

 olfactory nerve. It certainly is not without significance that 



VOL. 35, PART 1. — NEW SEB. ji 



