RELATION OF THE NEMERTEA TO THE VERTEBRATA. 637 



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 innerva- 

 tion of the respiratory portion of the intestine is brought 

 about — {a) by a pair of nerves directed backwards and spring- 

 ing from the anterior lateral swellings (the brain-lobes) of 

 the lateral nerve-stems; {b) by numerous visceral branches 

 starting from the plexus, directed inwards as branches that 

 spread over the wall of blood-lacunae 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. (1) the cephalic nerves, amongst which the 

 vagus nerve is in this respect the most important;^ (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 afterwards 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 



1 Ventrally these nerves (e. g. the u. hypoglossus) are sometimes commis- 

 surally united with their representative of the opposite haif of the body. It 

 must remain an open question whether these commissures are in any way 

 comparable either to the Nemertean vagus commissures (of. p. 83), or to the 

 general ventral commissural system of these worms. 



