REPORT ON THE NEMERTEA. 143 



The facts as tliey lie before us do not, however, admit of any very circumstantial com- 

 parison so far as the nerves in particular are concerned, and I purposely refrain from 

 entering into any details. Yet it should be remarked — 



(1) That the polymerous root of the Vertebrate vagus nerve is very readily explicable 

 if we take the Nemertean arrangement as a starting-point (PI. XVI. figs. 1, 2, vag), 

 as is also the mixture of sensory and motor elements in this root.' 



(2) That similarly, if the anterior cephalic nerves (e.g., the fifth) should prove to be 

 polymerous, this would in no way be astonishing nor difiicult to bring into harmony with 

 that same starting-point. 



(3) That the presence of superficial branches to the integument and to the muscu- 

 lature, and of deeper branches to the intestinal epithelium in those parts that will 

 contribute to form the cephalic nerves, is similarly foreshadowed in the Nemertea. 



(4) That the equivalent of the Nemertean vagus nerve will have to be sought for 

 in such branches of the Vertebrate vagus as more especially innervate the intestinal 

 epithelium,^ whereas the innervation of the Vertebrate gill-slits, which marks a later phylo- 

 genetic stage, in which these perforations of the anterior trunk region have appeared, 

 may be as well put to the account of more superficial parts of the transverse tracts. 



(5) That the common starting-point of the sensory, lateral, and the intestinal 

 portion of the vagus has also attracted the attention of former observers. Kansom and 

 d'Arcy Thompson write :- — " In the embryo dog-fish the second or ventral commissure 

 described by Balfour, &c., as uniting the roots of the vagus, ventral to the ganglia, is 

 essentially a sympathetic commissure, whose (visceral) fibres pass on, as described by 

 Balfour, to form the intestinal branch of the vagus. In that intestinal branch we have 

 an outflow of visceral fibres, quite comparable to, e.g. , a splanchnic branch of the dorsal 

 sympathetic system. The connection between the origin of the lateralis and this ventral 

 commissure connecting the vagus roots in the dog-fish, and similarly the relation of the 

 lateralis to the loops uniting the ganglia of the 5th, 7th, and 10th nerves in 

 Petromyzon may probably be described as indicating a fusion in this region of 

 the two great commissural systems which posteriorly are separate, viz., that of the 

 sensory branches (lateralis) and the visceral or sympathetic. 



' Rohon, Ueber den Urspruiig des Nervus vagus bei Selacliiern, Arbeit. Zool. Inst. Wien, vol. i. p. 159. 



^ I have good reasons, based upon actual observations made by my pupil, Mr. Dobberke, to believe that the ramus 

 intestinalis vagi in adult Elasmobranchs may lie traced centripetally from its region of innervation of the foremost portion 

 of the intestinal wall, towards the brain, as a bundle of nerve-tibres running parallel to and combined with those for the 

 branchial apparatus, but that, nevertheless, this bundle can be separately traced up to the vagus ganglion, without any 

 further intimate relation to those branchial branches (cf. Beard, loc. cit., p. 110). If this should^ actually be the case, 

 the possibility of a direct comparison between the Nemertean vagus nerve and the Vertebrate ramus intestinalis vagi, of 

 course, comes more closely within our reach. It need not be insisted upon that if these comparisons prove correct, the 

 separate intestinal nerve-systems (sympathetic nerve system) of other Invertebrates (Annelids, Arthropods, Molluscs) 

 camiot be looked upon as homologous with the sympathetic nerve system of the Vertebrates, but would rather be 

 homologous with that portion of the intestinal innervation of the latter which comes to the account of their cephalic 

 nerves, in so far as these represent derivatives of the Nemertean vagus, and are marked v in figs. 1 and 2 of PI. XVI. 



