272 THE PROTOCHORDATA. 



Vertebrates and the Invertebrates, are largely due to the 

 failure to detect some general principle of evolution to 

 which that archaic structure, the praeoral lobe, has been 

 subjected. 



Nevertheless, there are many indications which point 

 irresistibly to the conclusion, which I have recently 

 brought forward, that the prime factor which must be 

 recognised in the evolution of the praeoral lobe, from the 

 relations which it presents in the Invertebrates to those 

 which it holds in the Protochordates and Vertebrates, is 

 its emancipation from the central nervons system. 



In the great groups of the Annelids, Molluscs, and 

 Arthropods, the praeoral lobe (prostomium, procephalic 

 lobe) is essentially the seat of the brain or cerebral gan- 

 glion. The latter, through its representative, the apical 

 plate, is the main and often the sole element of the central 

 nervous system in the Trochophore-larva of Annelids and 

 Molluscs.* 



* In speaking of the apical plate as the forerunner or formative centre of 

 the cerebral ganglion, it must not be assumed that these are not distinct 

 structures. The apical plate is essentially median and unpaired, while the 

 cerebral ganglion is paired. They can both, however, be included under the 

 general term, apical nervous system, since they arise from the ectoderm of 

 the praeoral lobe. On the other hand, the cerebral ganglion may arise inde- 

 pendently of an apical plate; as, for instance, in Lunibriciis, where there is 

 no apical plate, or in the Nemertines, where the apical plate is discarded 

 together with other larval structures (Pilidium). Again, as in Lumbricus and 

 many other cases, the cerebral ganglion, after having separated from the 

 ectoderm of the praeoral lobe, may recede backwards for a considerable dis- 

 tance, so as not to lie in the prceoral lobe in the adult. It is possible that the 

 position of the cerebral ganglia of Nemertines may be accounted for by some 

 such phylogenetic recession from the prfeoral lobe. 



If necessary, it might be said that the proeoral lobe can acquire emancipa- 

 tion from the central nervous system by a simple recession of the cerebral 

 ganglion. In the case of the Protochordates, however, on the view here advo- 

 cated, the prjeoral lobe has acquired emancipation from the central nervous 

 system, not by the mere recession, but by the complete disappearance of the 

 Invertebrate cerebral ganglion. 



