The Enteropneusta especially fix our attention, because they alone, although essentially 'worms,' exhibit certain characteristics which make it possible to bridge over the gulf which still separates the Invertebrata from the vertebrate phylum. The anterior portion of the gut is transformed into a breathing apparatus—hence Gegenbaur's term of Enteropneusta, or Gut-breathers. Moreover, Balanoglossus and Cephalodiscus possess another modification of the gut—namely, a peculiar diverticulum, which, in the present state of our knowledge, may be looked upon as the forerunner of the chorda dorsalis.
9. Stage of Prochordonia, as indicated by the larval form, called Chordula, which is common to the Tunicata and all the Vertebrata. These two groups possess three most important features: (a) A chorda dorsalis, a stiff rod lying in the long axis of the body, dorsally from the gut and below the central nervous system. This latter, for the first time in the animal kingdom, appears in
