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



observed it "bis an das Hinterende des Korpers") is not confirmed by modern investi- 

 gators. Ahlborn's description (loc. cit., p. 304) of the variable situation of this nerve in 

 Petromyzon is very suggestive in connection with the views here advocated. Eansom 

 and d'Arcy Thompson consider that the regularity of the integumentary sensory 

 apparatus is not yet established in Petromyzon, as may be concluded from the citation 

 given above (p. 140). 



We have now considered the superficial ramifications of what I may call the lateral 

 nerve system, both in lower worms and in Vertebrates ; we must now turn to the 

 intestinal, to the visceral branches of this same system, from which other and important 

 data may be gathered for further elucidation of the hypothesis under consideration. 



We have already seen that in Nemertea the typical innervation of the respiratory 

 portion of the intestine is brought about — (a) by a pair of nerves directed backwards and 

 springing from the anterior lateral swellings (the brain-lobes) of the lateral nerve-stems ; 

 (Jj) by numerous visceral branches starting from the plexus, directed inwards as branches 

 that spread over the wall of blood-lacunas and intestine. 



In the Vertebrata, Amphioxus excepted, we also find that the innervation of the 

 anterior respiratory portion of the intestine and of the circulatory apparatus is obtained 

 from two sources, viz., (l) the cephalic nerves, amongst which the vagus nerve is in this 

 respect the most important 1 ; (2) the visceral branches of the spinal nerves, which are at 

 the basis of what is afterwards more highly differentiated and separately recognised as 

 the sympathetic nerve-system. 



In Nemertea it is very difficult to determine in the anterior part of the intestinal 

 wall, which tracts belong to the so-called vagus nerve, which to this system of visceral 

 nerve-branches. 



So it is often in Vertebrata, and the blending together (in both divisions of the 

 animal kingdom) of two systems, each of them again mutually comparable when separately 

 considered, is an important point of agreement, and would, if no actual homology were 

 at the base of it, be a very puzzling coincidence. 



It is in this respect highly suggestive that Born notices, as early as 1827, what was after- 

 wards confirmed by Ahlborn (loc. cit) and others, that in Petromyzon, i.e., one of the lowest 

 Vertebrates, the spinal nerves send out connecting branches towards the pneumogastric 

 nerves. The existence of superficial metameric connections (Eansom and d'Arcy Thompson, 

 vide supra) as well as of this set of deeper connections between the transverse and the 

 latero-longitudinal nerve-stems (n. lateralis and n. pneumogastricus) of Petromyzon would 

 thus be a most remarkable repetition of the similar arrangement in the Nemertea, 

 as it has been here for the first time demonstrated. 



1 Ventrally these nerves (e.g., the n. hypoglossus) are sometimes commissurally united with their representative of 

 the opposite half of the body. It must remain an open question whether these commissures are in any way comparable 

 either to the Nemertean vagus commissures (c/. p. 83), or to the general ventral commissural system of these worms. 



