Evolution of Fishes from Invertebrates 79 



the posterior end of the body, the two nerves gradually rise 

 upward and form a posterior commissure above the intes- 

 tine. Now were such a close connection to be gradually 

 carried forward toward the brain region, the result would 

 be that an apposition and ultimate more or less complete 

 dorsal fusion would be effected between the two lateral 

 nerves. 



But in rhabdocoel turbellarians and in metanemerteans 

 two dorsal nerve threads gradually come together and fuse 

 lengthwise as the median dorsal nerve of the latter group. 

 This nerve, according to most authors, springs from the 

 dorsal brain commissure, and there arises by two distinct 

 roots. Now in the vertebrate brain a fine nerve arises in 

 median line from the anterior commissure but by two roots. 

 This, first observed about sixty years ago by Reissner, has 

 been traced to occur in the entire vertebrate series by a 

 number of workers. Its main investigator Sargent {^3: 

 129) states that it tends to suffer degeneration only in 

 animals like the blind cave fishes, where the eyes have more 

 or less become functionless or absorbed. In its backward 

 passage from the brain this nerve fibre becomes embedded 

 between the substance of the lateral thickenings of the spinal 

 cord, and tapers out near the anus.* 



Further, in rhabdocoels and in worms as a great group, 

 two ventral nerve threads start from the ventral gan- 

 glia of the head and run backward, either widely apart, or 

 somewhat near each other, or closely apposed. These, in 

 the more evolved "worms" and in "arthropods," become 

 the dominant nerve tracts, while the lateral and the dorsal 

 pairs of nerves become feeble or are absorbed. In meta- 

 nemerteans these ventral threads are moderately represent- 

 ed as median nerves of the lower surface. So in these latter 

 account has to be taken of three pairs of longitudinal nerves, 

 as a whole: (i) the ventral median nerves that are only 

 moderately developed but have become the main ones in 

 worms and in arthropods; (2) the lateral or latero-dorsal 

 nerves that are the specially strong ones in metanemer- 



*The writer is unable to accept the views of Nicholls on this subject (46: 1). His 

 conclusion that the median dorsal thread is a muscle, is entirely uncorrelated with any 

 like structure in other animals. His paper throughout is characterized by a malignant 

 and unscientific spirit that is wholly regrettable. 



