RELATION OF THE NEMEBTEA TO THE VERTEBRATA. 639 



the innervation of the Vertebrate gill-slits, which marks a later 

 phylogenetic 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. Ransom and d'Arcy Thompson 

 write : ^' In the embryo dog-fish the second or ventral com- 

 missure, described by Balfour, &c., as uniting the roots of the 

 vagus, ventral to the ganglia, is essentially a sympathetic com- 

 missure, 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 tlie relation of the lateralis to the loops uniting 

 the ganglia of the fifth, seventh, and tenth nerves in Petro- 

 my z on, 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. 



" We agree with Gaskell that the term sympathetic should 

 be suffered to fall into disuse, as tending to perpetuate the old 

 conception of the primary importance of the longitudinal 

 nerve-tract; whereas the leading fact is the metaraerically 

 recurring outflow of visceral fibres, which may or may not be 

 united together by successive longitudinal commissures.'^ 



In the Nemertea this anterior " fusion of the great com- 

 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 s,eparate intestinal nerve-systems 

 (sympathetic nerve system) of other Invertebrates (Annelids, Arthropods, 

 Molluscs) cannot be looked upon as homologous with the sympathetic uerve- 

 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 fi^s. 1 and 2 of PI. XLII. 



