12 



THE ORIGIN OF VERTE B RATES 



It has been supposed that the Tunicata and the Enteropneusta 

 {Balanoglossus) (Fig. '2) are members of this missing chain, and that 



in Amphioxus the ver- 

 tebrate approaches in 

 organization to these 

 low invertebrate forms. 

 The timicates, indeed, 

 are looked upon as de- 

 generate members of an 

 early vertebrate stock, 

 which may give help in 

 picturing the nature of 

 the vertebrate ancestor 

 but are not themselves 



in the direct line of 

 Fig. 2.— Larval Balanoglossus (from the Royal 



Natural History). descent. Balanoglossus 



is supposed to have 

 arisen from the Echinodermata, or at all events to have affinities 

 with them, so that to fill up the enormous gap between the 

 Echinodermata and the Vertebrata on this theory there is absolutely 

 nothing living on the earth except Balanoglossus, Bhabdopleura, 

 and Cephalodiscus. The characteristics of the vertebrate upon 

 which this second theory is based are the notochord, the respiratory 

 character of the anterior part of the alimentary canal, and the tubular 

 nature of the central nervous system ; it is claimed that in Balano- 

 glossus the beginnings of a notochord and a tubular central nervous 

 system are to be found, while the respiratory portion of the gut is 

 closely comparable to that of Amphioxus. 



The strength of the first theory is essentially based on the com- 

 parison of the vertebrate central nervous system with that of tho 

 segmented in vertel irate, annelid or arthropod. In the latter the 

 central nervous system is composed of — 



1. The supra-cesophageal ganglia, which give origin to the nerves 

 of the eyes and antennules, i.e. to the optic and olfactory nerves, 

 for the first pair of antenna? are olfactory in function. These are 

 connected with the infra-cesophageal ganglia by the oesophageal 

 commissures which encircle the oesophagus. 



2. The infra-cesophageal ganglia and the two chains of ventral 

 ganglia, which are segmentally-arranged sets of ganglia. Of these, 



