Evolution of Animals 437 



got under the sixth caption, and this in the following manner. 

 In connection with captions 2 and 4, as gradual apposition 

 and more or less perfect union of the ganglionic or brain masses 

 and nerve cords took place, one of three "fates" might also 

 have happened to the median dorsal nerve. Either it might 

 have risen above the apposing masses to occupy a free position 

 dorsal to them, or it might have become included in the cavity 

 that resulted from their apposition and union, or it might 

 have transferred its functions to another nerve and become 

 obliterated. The second is clearly the result that has been 

 worked out, and as now existing in vertebrates the dorsal 

 nerve is known as Reissners fiber, though retaining its primi- 

 tive origin, distribution, and function (Fig. 18, c-e). 



Had Hubrecht's attention been turned to this structure of 

 the vertebrate brain, such would at once have confirmed him 

 in his and in Harting's original contention, and furnished 

 his most powerful argument for derivation of the vertebrates 

 from nemerteans. For, discovered by Reissner in 1860, within 

 the neural canal of Petromyzon, it was subsequently observed 

 by Kutschin, Stieda, Sanders, and Studnicka in Petromyzon, 

 selachians, teleosts, and by the last even in amphibians and 

 mammals. But the most extensive and accurate investi- 

 gation of it has been made by Sargent {lJf-2: 129), who also 

 gives a full bibliography. He has further added descriptive 

 details that strengthen some of the above six captions. 



Sargent shows (p. 139) that, while cavities or ventricles of 

 the central nervous system are of small size in higher verte- 

 brates, in the lower and specially in Petromyzon and Myxine 

 the cavities or ventricles are large. Such would be appropri- 

 ate evolutionarily, if the vertebrate brain has resulted from 

 convergence of nemertean ganglionic lobes. 



Next, the origin of the dorsal nemertean nerve and of Reiss- 

 ner's fiber from the connecting commissure of the opttco- 

 olfactory areas is identical. Reference to Figure 17b shows 

 the origin of the dorsal nerve in nemerteans midway between 

 the pairs of optic and olfactory nerves. In this connection 

 Sargent (p. 162) says: "It is evident that in the cyclostomes, 

 where the eyes are poorly developed, Reissner's fiber serves 

 as a short circuit for the transmission of reflexes arising from 

 olfactory as well as optic stimuli." 



In histological detail as given respectively by Burger (13^: 

 108) and Sargent (p. 145) the resemblance is marked in both 

 groups of animals. That it is a structure also that has become 

 overshadowed by a more powerfully developed system, and 

 rather readily tends to disappear when its function as a lumino- 



