REPORT ON THE NEMERTEA. 135 



The opposite half of the system, seen in transparent perspective, as given in the two 

 other figures, is purposely omitted here, because of the asymmetry of AmpMoxus in 

 this respect. 



Now a glance at these figures will convince us that the situation of the Nemertean 

 medullary nerve in its plexus, and with its set of transverse nerves, is directly compar- 

 able to the Vertebrate medulla and spinal nerves. The nerve-plexus filling up the 

 intervening spaces in Nemertea is present as a transitory structure in Amphibian 

 embryos. 



The ulterior appearance of an anterior enlargement forming the Vertebrate brain ; 

 the higher complication attained by the brain and spinal cord when its mass increases, 

 but not its dorsal expansion, by the appearance of medullary ridges ; and the formation of 

 a neural canal by infolding of the neural plate, all these are important developmental 

 facts which do not in any way weaken the grounds for comparison of the two structures. 

 They may be looked upon as adaptations to the much more considerable efiiciency 

 and concentration that is gradually attained by the central nervous system as we ascend 

 higher in the scale of the animal kingdom. 



The fact that the neural ridge in so many Vertebrata precedes the appearance of the 

 spinal nerves, and is inserted along the top of the folds that bend together to form 

 the neural tube, may be thus interpreted, that during the phylogenetic process 

 of infolding, the transverse nerve-tracts (dorsal spinal roots) remain attached in the 

 same way to the medio-dorsal collecting trunk as they did in the ancestral forms, and 

 are dragged upwards by the infolding process. The ventral roots must be phylo- 

 genetically linked to the plexus as well ; 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 coincidence between Ainphioxus 

 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 portion 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 be again insisted on, 

 now that the researches of Eohon,^ Freud,^ Schneider, Ransom, and d'Arcy Thompson' 



^ v. Eolion, Untersuchungen iiber Amphioxus lanceolatus, Denksch: d. k. Ahai. d. JFus. Wien, Bd. xlv. 



- S. Freud, Ueber Spinalganglien und Riickenmark des Petromyzoii (Sitxungsb. niath.-nat. cl. k. Akad. Wiss. Wien^ 

 Bd. Ixxviii., Abth. 3, 1878). This author says (p.l54) : " Ich kann wenigstensvon den letzten Wurzeln des Caudalmarks 

 sagen dass ihre Selbstandigkeit so gross ist, dass man von vorderen uud hinteren Spinalnerven, anstatt von vorderen 

 und hinteren Wurzeln reden konnte " ; and Wiedersheim in the 2d edition of his Lehrbuch der Vergleichenden 

 Anatomic (p. 321): "Vieles spricht dafiir dass die Vorfahren der heutigen Wirbelthiere getrennte dorsale und 

 ventrale Nerven wurzeln besessen haben miissen." 



3 W. R. Ransom and d'Arcy W. Thompson, On the spinal and visceral nerves of Cyclostomata, Zool. Anzeiger, No, 227, 

 July 1886. 



