144 THE VOYAGE OF H.M.S. CHALLENGER. 



" We agree with Gaskell that the term sympathetic should be suffered to fall iuto disuse, 

 as tending to perpetuate the old conception of the primary importance of the longitudinal 

 nerve-tract ; whereas the leading fact is the metamerically 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 commissural systems " is 

 foreshadowed at the point where brain-lobe, lateral stem, and "vagus nerve " meet, or 

 rather diverge. It has been attempted in figs. 1 and 2 to indicate the points here 

 alluded to in a general way, special comparisons being, on the grounds that have been 

 stated, purposely avoided. 



If we now turn to Dohrn's and Semper's hypothesis we must recognise that no such 

 satisfactory general comparisons are there possible. Even if we were inclined to accept 

 the " turning over " of Geoffroy St. Hilaire, by which back and belly became exchanged, 

 and to admit the brain-piercing oesophagus, regarding the Annelid supracesophageal 

 ganglion and the ventral nerve-cord as respectively homologous to cerebrum and 

 medulla, it must still be conceded that we have not then in any way before us a nerve 

 system offering as many points of comparison with the Vertebrate system as does that of 

 the Nemertea. 



Concerning the Annelids we have no observations by which the cephalic ganglia and 

 the cephalic nerves are so clearly foreshadowed, none which would throw light on the origin 

 of the vagus, its connection with the nervus lateralis and with the anterior cephalic 

 ganglia, none concerning the sympathetic system and its blending with the vagus system 

 in the lowest Vertebrates, indications of which are even retained in the highest. Nor is 

 the ventral nerve-cord of Annelids, with its undeniable double character and double 

 origin a match, so far as comparison goes, for the Nemertean medullary nerve, with its 

 transverse nerves preceding the spinal nerves of Ampliioxus and the Cyclostomata. 



And if we are then asked to consider the lens of the Vertebrate eye as a modified 

 ectodermal branchial invagination, as the outer portion of what was once a functional 

 gill-slit, 1 we feel that the ground under our feet is becoming rather uncomfortable, and 

 that it is high time to reconsider whether all these ingenious speculations in which the 

 most beautifully pliable hypothetical and unknown Annelids play a too conspicuous part 

 should not be definitely abandoned, and a new departure made by those who are interested 

 in the phylogeny of the Chordata. In due time arduous and detailed morphological 

 investigations on the Platyelminthes in general, and on the Nemertea in particular, may 

 then lead us to more satisfactory conclusions than are the fata morgana that are so 

 temptingly evoked before our eyes by the ingenious manipulations of the indefatigable 

 founder of the first and foremost Zoological Station, when, following his lead, we find 

 ourselves wandering in the barren deserts of that province of phylogeny, in which he 

 attempts to establish a close connection between Chordata and Annelida. 



1 Dobrn, Studien, x. p. 459, 1885. 



