454 THE ORIGIN OF VERTEBRATES 
Finally, with the conversion of this groove into a tube, the opening 
of the oral into the respiratory chamber, and the formation of an 
atrium by the ventralwards growth of the pleural folds, the formation 
of a Vertebrate was completed (Fig. 167, D). 
In my own mind I picture to myself an animal which possessed 
eurypterid and trilobite characters combined, in which a notochordal 
tube had been formed in the way suggested, and a respiratory chamber 
which communicated with the cloaca by means of a grooved channel 
along the mid-ventral line of the metasomatic portion of the body. 
On each side of this channel were the remains of the metasomatic 
appendages (pronephric). The whole was enveloped in the pleural 
folds, which probably at this time did not yet meet in the middle 
line to form a new ventral surface. This respiratory chamber, owing 
to the digestive power of the epidermis, assisted in the process of 
alimentation to such an extent as to supersede the temporary noto- 
chordal tube, with the effect of bringing about the conversion of the 
metasomatic groove into a closed canal, and so the formation of an 
alimentary tube continuous with the respiratory chamber. The 
amalgamation of the pleural folds ventrally completed the process, 
and so formed an animal resembling the Cephalaspide, Ammocctes,. 
or Amphioxus. 
I have endeavoured in this chapter to make some suggestions 
upon the origin of the notochord and of the vertebrate gut in accordance 
with my theory of the origin of vertebrates. I feel, however, strongly 
that these suggestions are much more speculative than those put 
forward in the previous chapters, and of necessity cannot give the 
same feeling of soundness as those based directly upon comparative 
anatomy and histology. Still, the fact remains that the origin of the 
notochord is at present absolutely unknown, and that my speculation 
that it may have originated as an accessory digestive tube is at all 
events in accordance with the most widely spread opinion that it 
arises in close connection with an alimentary canal. 
