12 THE ORIGIN OF VERTEBRATES 
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 tunicates, 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 
Fic. 2.—Larvan Batanociossus (from the Royal in the direct line of 
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, Rhabdopleura, 
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 the 
segmented invertebrate, 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 antenne are olfactory in function. These are 
connected with the infra-cesophageal ganglia by the cesophageal 
commissures which encircle the cesophagus. 
2. The infra-cesophageal ganglia and the two chains of ventral 
ganglia, which are segmentally-arranged sets of ganglia. Of these, 
