THE ANCESTRY OF THE VERTEBRATES 521 



into the various ganglia found in vertebrates, in association 

 with the cranial nerves. As for the commissural nerves, they 

 become alternately sensory and motor in function and associate 

 together in pairs, forming the metamerically arranged spinal 

 nerves of vertebrates, the elements of which are in lower forms 

 still separate and issue from the neural canal through separate 

 foramina. Lastly a sympathetic system is formed by collect- 



B 



FIG. 141. Nemertean theory of the origin of Vertebrates. [After 

 HUBRECHT.] 



(A) Typical diagram of Nemertean. d, dorsal nerve cord; gl, ganglion; It, 

 lateral nerve cord; v, intestinal nerve; sb, small intestinal branches. 



(B) Typical diagram of Vertebrate; db, brain; d, dorsal nerve cord; s, sensory, and 

 m, motor spinal nerves; gl, sympathetic ganglia; v, ramus intestinalis vagi; It, ramus 

 lateralis vagi; sb, sympathetic branches. 



ing together the small intestinal branches that come from the 

 ventral portions of the commissural nerves. 



Concerning the other systems it is only fair to say that their 

 correspondence is by no means as close as is that of the nervous 

 system, although a characteristic nemertean structure, the 

 proboscis, has been likened to the hypophysis, while its sheath, 

 into which it may be retracted, has been cited as possibly fur- 



