Evolution of Fishes from Invertebrates 77 



anteriorly from it along the rim region (rhynchodoeum) 

 would be effected. Thus the way would be clear for the 

 ventral brain ganglia, and for the ventral commissure that 

 united these to rise closely upward and in front of the 

 blunt anterior end of the evolving notochord, till placing 

 of the ventral ganglia, somewhat behind, but close against 

 the dorsal ganglia would be effected. 



The writer then, in view of this and the many other 

 details of structural similarity that he now shows to exist, 

 would emphatically advance on Hubrecht's decidedly cau- 

 tious position when he says: "I need hardly insist upon the 

 fact that I do not advocate any direct relation between 

 existing nemerteans and existing vertebrates; my argument 

 goes no further than the attempt to show that the general 

 plan of structure of a nemertean is more in accordance with 

 that of a vertebrate animal than is, for example, that of 

 the Archiannelida." So many exact structures correspond 

 in both groups, or are carried forward from a more prim- 

 itive nemertean to a more evolved chordate state, that 

 the writer would claim a continuous evolutionary sequence, 

 even though some desiderated and highly important inter- 

 mediate stages are still wanting. 



IX, The Nervous System. 



This eminently confirms the above conclusions. The 

 writer has already dealt at considerable length with it (/:- 

 433-441 ), but some details deserve emphasis here. 



The brain in most metanemerteans consists of two rela- 

 tively large dorsal or anterior brain-lobes, that are united 

 by a dorsal commissure. The latter, from its apparent 

 homology with vertebrate details, we can call the anterior 

 commissure. In nemerteans and in fishes the writer has 

 pointed out that a median nerve starts from the middle 

 of the anterior commissure. In nemerteans and in the 

 simpler turbellarians, zoologists have for years called this 

 the dorsal nerve. But in fishes and higher vertebrates it 

 has been called from its discoverer Reissner's fibre. It 

 is again referred to below. 



While the anterior or dorsal brain-lobes remain simple 

 in most of the metanemerteans, in the genus Eupolia at 



