PRSEORAL LOBE. 2?$ 



At a later stage of development the longitudinal nerve- 

 cord (confining the description to the Annelids for the 

 sake of simplicity) arises independently of the cerebral 

 ganglion, from a pair of longitudinal thickenings of the 

 ectoderm near the mid-ventral line, becoming secondarily 

 connected with the cerebral ganglion by the circumoesoph- 

 ageal nerve-collar or commissure. 



As already indicated, it seems probable, as was sug- 

 gested by BALFOUR and GEGENBAUR, that the ventral 

 nerve-cord of the Annelids is to be regarded as having 

 arisen phylogenetically by the mutual approximation of 

 two such lateral cords as occur in the Nemertines, and 

 like the latter may be supposed to have originated by a 

 concentration on the ventral side of the body of that 

 primitively continuous sub-epidermic nerve-plexus which 

 is such a characteristic feature of the Nemertines. From 

 a consideration of the adult nervous system in the 

 Echinoderms, Nemertines, Enteropneusta (Balanoglossus), 

 Annelids, and Molluscs, it is evident that such a con- 

 centration of nervous tissue has from first to last occurred 

 along very different lines. 



Speaking in broad terms, it may be said that the only 

 portion of the Invertebrate nervous system which, in its 

 prime essence, is invariable and universal (due allowance 

 being made for exceptional cases) is the cerebral ganglion 

 or its forerunner, the apical plate, the seat of which lies in 

 the praeoral lobe. 2 



Under these circumstances it will suffice to confine our 

 attention to the prasoral lobe, in the belief that if an 

 understanding can be arrived at with regard to that impor- 

 tant structure, one of the chief difficulties in the way of a 

 just conception of the relations existing between Verte- 

 brates and Invertebrates will have been overcome. 



