/'A'.7;\)A'.-//. LOBE. 



273 



At a later stage of development the longitudinal nerve- 

 eord (eontiiiing the deseription to the Annelids tor the 

 sake ot simplieity) arises mdc-prndcut'.y of the eerebral 

 ganglion, from a pair of longitudinal thiekenings of the 

 eetoderm near the mid-\-entral line, beeoming seeondarily 

 eonneeted with the eerebral ganglion bv the cireuma;soph- 

 ageal nerve-eollar or eommissure. 



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

 gested by Balfour and Gkok\1!auk. that the ventral 

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

 arisen phylogenetically b_\- the mutual approximation of 

 two sueh lateral eords as oceur in the Xemertines, and 

 like the latter mav be supposed to ha\'e originated bv a 

 eoneentration on the ventral side of the bodv of that 

 primitively eontinuous sub-epidermie nerve-plexus whieh 

 is sueh a eharaeteristie feature of the Nemertines. From 

 a eonsideration of the adult nerwnis svstem in the 

 Hchinoderms, Nemertines, linteropneusta (lialanoglossus\ 

 Annelids, and Molluses, it is evident that sueh a eon- 

 eentration of nervous tissue has from tirst to last oeeurred 

 along \-ery different lines. 



Speaking in broad terms, it mav be said that the onlv 

 portion of the Invertebrate nerx'ous s\-stem whieh, in its 

 prime essenee, is invariable and UTiiversal (due allowanee 

 being made for exeeptional cases) is the eerebral ganglion 

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

 the praxiral lobe.- 



Lender these circumstances it will suffice to confine our 

 attention to the pra^^ral lobe, in the belief that if an 

 understanding can be arri\ed at with regard to that impor- 

 tant structure, one of the chief difficulties in the wa\' of a 

 just conception of the relations existing between \'erte- 

 brates and In\-ertebrates will have been overcome. 



