THE ORIGIN AND EVOLUTION OF THE VERTEBRATES 



83 



system. The nemertean (Fig. 45) has two lateral nerve cords and a 

 more slender dorsal nerve cord. The three cords are connected with 

 one another by cross commissures. It is thought that in the evolution 

 of the Nemertea into vertebrates the dorsal nerve cord becomes the 

 central nervous system and is then enlarged at the anterior end into 

 a brain ; while the two lateral nerve cords become compatively small 



Fig. 45. — To illustrate the Nemertean Theory of the origin of the Vertebrates. 

 A. A typical diagram of nemertean. d, dorsal nerve cord; gl, ganglion; ll, lateral 

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



B. Typical diagram of vertebrate, db, dorsal brain; d, dorsal nerve cord; s, 

 sensory, and m, motor spinal nerves ; gl, sympathetic ganglia ; ).), ramus intestinalis 

 vagi; sb, sympathetic branches. (From Wilder, after Hubrecht.) 



and of secondary importance, persisting however in the so-called 

 "Vagus system, rami lateralis X" in the lower vertebrates. It can 

 scarcely be said that this theory carries force in view of the fact that 

 the other structures of the body show little resemblance to vertebrate 

 conditions. 



Another theory of vertebrate ancestry associates the latter with the 

 Echinodermata through the connecting link Enteropneusta (Balan- 

 oglossus). The Balanoglossus situation is indeed a puzzling one. 

 It does not seem to fit in with any of the other theories of vertebrate 



