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 Cephalaspidse, Ammoccetes, 

 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. 



