MORPHOLOGY OF BRAIN AND SENSE ORGANS OF LIMOLUS. 75 



It is evident that the principal difterences between the fore- 

 brain of Limulus and that of Vertebrates is a diflFerence in 

 degree and not in kind. In Limulus the infoldings are obscurCj 

 in not always showing the duplication of layers ; individualized, 

 in that in each important brain-lobe they progress indepen- 

 dently of the others ; and they are incomplete, in that they do 

 not always unite so as to entirely enclose the infolded parts. 

 Moreover some important cavities, such as the lateral ventricles 

 and the cavities in the optic ganglia and infundibulum, disap- 

 pear at an early stage, so that their relations to the permanent 

 cavities are not very obvious. In Vertebrates, on the contrary, 

 the infoldings are very simple because they have lost their in- 

 dividuality by fusing into one continuous fold, which appears 

 very early, and is usually completed before the brain-lobes are 

 specialized. In Limulus the phylogenetic processes from the 

 very first stages dominate over the pure mechanics of ontogeny. 

 In Vertebrates it is almost the reverse; purely embryological 

 processes prevail at first, afterward phylogenetic ones are 

 manifest. 



There is no greater difiiculty in identifying all the charac- 

 teristic features of the Vertebrate brain in Limulus than there 

 is in comparing a fish brain with that of mammals. No other 

 Invertebrate will permit any approach to such comparison. 

 On the other hand, it is impossible that the comparisons, such 

 as I have instituted, could be carried so far without breaking 

 down, unless they rested on a sure foundation of actual facts 

 and correct premises. 



It remains now to say a few words concerning the parietal 

 eye and the olfactory organ. If the relation of the parietal 

 eye to the primary cerebral vesicle and to the infundibulum are 

 mere coincidences, having nothing to do with the genetic re- 

 lationship of Limulus with Vertebrates, then in all probability 

 the resemblance will cease there ; it certainly cannot extend 

 any farther, say to the structural details of its nerves and its 

 terminal portions. But the resemblance does go a great deal 

 farther, as we shall now see. 



