EYES OF INTERMEDIATE TYPES 169 



elusions of Bateson and others that they were derived from and represented 

 the parent stock, which had given rise to the whole vertebrate kingdom. 

 Later workers, including Ritter and MacBride, have, however, re-affirmed 

 an important item in the original thesis of Bateson, namely that bilateral 



I--"- gen- 



nhep. 



• ■ -i 



Fig. 123. — Balanoglossus. (After Spengel, from Parker and Haswell's 



Textbook of Zoology.) 

 br. : branchial regions. 

 co. : collar. 

 gen. : genital ridges. 



hep. : projections caused by a series of paired hepatic pouches. 

 pr. : proboscis. 



segmented animals have evolved in two directions — one, in which the 

 alimentary canal pierces the nervous system between the supra-oesophageal 

 ganglion and the infra-oesophageal ganglion, leading up to the annelids 

 and arthropods ; and the other, in which the central nervous system is 

 dorsal to the alimentary canal, from which the protochordata and verte- 



