438 Causes and Course of Organic Evolution 



muscular correlator becomes reduced or superfluous, is sIioami 

 by Sargent's study of such blind fishes as Amblyopsis and 

 allies, in wliicli it may suffer varying degrees of degeneration 

 to the point of ultimate disappearance. 



The quite unique position and course of Reissner's fiber 

 can only be explained by its ha^dng become surrounded and 

 included by two longitudinal cordal structures, that primi- 

 tively were more ^^'idely apart. Its continued presence and 

 structural similarity, even up to the mammals, are probably 

 explained by its having retained its functional activity through- 

 out the entire scale from turbellarians and through the nemer- 

 teans upward to mammals. As in some rhabdocoels, and 

 apparently also in nemerteans, to judge from illustrations, 

 so in vertebrates, Reissner's fiber arises from two roots, but 

 tliese remain separate only in rhabdocoels. 



The writer would therefore regard the structural, the anatom- 

 ical, and the physiological evidence above cited as convincing 

 proof that a direct connection exists between the nemertean 

 and cyclostome brain systems, and in epitome he would now 

 outline the probable evolution of the vertebrate brain and 

 associated parts from that of nemerteans as follows. 



In primitively fresh-water types allied to the existing Geo- 

 nemertes, a pair of dorsal ganglia, hereditarily related to those 

 of rhabdocoels and rotifers, was formed by dorsal involution 

 of ectoderm. These were connected by a dorsal commissure 

 that became the posterior commissure of the vertebrate brain, 

 and from this arose a dorsal nerve that represented the fused 

 dorsal nerves of rhabdocoels, and which in nemerto-vertebrates 

 t)ecame Reissner's fiber, that still arises by double roots. The 

 antero-inferior parts of the dorsal ganglia gave off olfactory 

 nerves to a single olfactory body that remains such in 3Iyxine, 

 and probably was also in primitive cyclostomes. But in Pefro- 

 myzon and higher vertebrates it has divided into two olfactory 

 sacs. The antero-superior parts of the ganglia gave off branches 

 to the two paired eyes, and to the two unpaired antero-posterior 

 eyes seen in many metanemertines. Commissural threads, 

 alike from the olfactory and optic nerves, crossed the dorsal 

 commissure, and passing into the dorsal nerve enabled the 

 latter to send correlated olfactomotor and opticomotor stimuli 

 to the anterior skin muscles. This function according to 

 Sargent, is continued in what the writer would regard as its 

 hereditary derivative, Reissner's fiber. 



A tendency to division of the two dorsal ganglia or brain 

 masses into an anterior and posterior pair of lobes, as in Eupolia, 

 occurred, and then from the latter pair nerve branches sprang 



