RELATION OP THE NEMERTEA TO THE VEETEBRATA. 627 



inasmuch as the musculature originally lies inwards of the 

 nervous plexus, their deeper situation is not surprising. 



In the points hitherto enumerated there is entire coinci- 

 dence between Amphioxus and the other Vertebrata, as far 

 as their comparability with the Nemertean diagram goes. 

 Another point of coincidence is the way in which the foremost 

 position of the intestinal canal and adjacent blood-vessels are 

 innervated by visceral nerve-stems^ indicated in all the three 

 diagrams by vi. sy. 



The claims to validity of the comparison here made between 

 the spinal nerves of the Chordata and the transverse stems 

 of the Nemertea should again be insisted on, now that the 

 researches of Rolion,^ Freud/ Schneider, Ransom, and d'Arcy 

 Thompson^ have established for the lower Chordata (Cephalo- 

 chorda and Cyclostomata) that the typical chordate spinal 

 nerve is not originally provided with a double root, but that 

 this double root appears to have arisen by the coalescence of 

 what were primitively in the groups just mentioned separate 

 and alternating dorsal and ventral nerve-tracts. With these 

 so much simpler spinal nerves the transverse nerve-stems of 

 the Nemertea undoubtedly offer points of comparison. These 

 Nemertean nerves specially differ from the Vertebrate spinal 

 nerves in two respects : (1) they give off nerve-fibres in differ- 

 ent directions, which are probably motor as well as sensory 

 and visceral, according to the different organ systems they 



1 V. RohoD, " Untersucbungen uber Amphioxus lanceolatus," 

 ' Denkschr. d. k. Akad. d. Wiss. Wien,' Bd. xlv. 



- S. Freud, " Ueber Spinalganglieii und Riickenmark des Petromyzou 

 ('Sitzungsb. niath.-nat. cl. k. Akad, Wiss. Wien,' Bd. Ixxviii, Abtb. 3, 1878). 

 This author says (p. 154) : — "Ich kann wenigstens von den letzten Wurzeln 

 des Caudalmarks sagen dass ihre Selbstandigkeit so gross ist, dass man von 

 vorderen und hinteren Spinalnerven, anstatt von vorderen und hinteren 

 Wurzeln reden konute " ; and Wiedersheim in the 2nd edition of his ' Lehr- 

 buch der Vergleichenden Anatomic ' (p. 321) : — " Vieles spricht dafiir dass die 

 Vorfahren der heutigen Wirbellhiere getrennte dorsale und ventrale Ner- 

 venwurzeln besessen haben miissen." 



^ W. E.. Ransom and d'Arcy W. Thompson, " On the Spinal and Viscera 

 Nerves of Cyclostomata," ' Zool. Anzeiger,' No. 227, .Tuly, 1886. 



