440 Causes and Course of Organic Evolution 



The originally strong lateral or dorso-lateral nerves, that 

 are a continuation backward of the ventral ganglia, by gradual 

 upward approximation, as seen proceeding in the nemertean 

 genus Langia, ultimately united to constitute the spinal cord 

 of vertebrates. 



The rather strong paired nerves described by Burger as 

 starting from the ventral ganglia, and named by him "mund- 

 schlund nerven" and "russel nerven," seem well to correspond, 

 in their origin and distribution, with two important systems 

 of nerves in vertebrates. The "mund-schlund" system, from 

 its origin along the lower posterior edges of the ventral brain- 

 masses, and its abundant distribution round the mouth, the 

 pharynx, the alimentary canal, and the skin, would correspond 

 with the ventral nerve threads of turbellarians. This seems 

 gradually to have evolved into what might be termed the 

 vent ro-sympathe tic in protochordates, and later into the sym- 

 pathetic system of cyclos tomes and higher vertebrates. The 

 latter or "rlissel" system specially and richly innervates the 

 entire proboscis-tube area and the anterior part of the ali- 

 mentary canal. Unfortunately, little seems to be known of its 

 complete and ultimate ramifications, specially toward the 

 posterior part of the nemertean body. The entire system 

 however suggests homology with the glossopharyngeal and 

 the vagus nerves. 



A detailed morphological and physiological investigation of 

 both systems in some of the larger land nemerteans is highly 

 desirable. More careful study also of the ventral ganglia 

 of the Metanemertinea, alike morphological and physiological, 

 may yet reveal evolving but exact rudiments of the fifth, sixth, 

 and seventh nerves. 



The numerous points of morphological continuity thus indi- 

 cated furnish strong proof that the nemertean brain passed 

 by progressive evolution through a nemerto-vertebrate stage 

 to that seen in cyclostome or allied but now extinct types. 



It may correctly be said then that the vertebrate brain, 

 as foreshadowed in that of the higher nemerteans, consists 

 df an anterior olfacto-sensory lobe, the prosencephalon; of a 

 median optico- (or helio-) sensory lobe, the mesencephalon; 

 and of a conjoint aiidiio-geotactic-sensory portion, derived in 

 l)art from the last and in part from the myelencephalon. 

 Further, that the diffuse media notacticy therrnotactic, hydro- 

 tactic, and chemotactic (gustatory, etc.) sense perceptions were 

 concentrated in the latero- ventral lobes that became the myel- 

 encephalon. Or, in other words, of the eight most important 

 animal senses or perception centers, four became localized 



