Conclusion.
In conclusion, I will endeavour to illustrate crudely the way in which, on my theory, the notochord and vertebrate gut may have been formed, the agencies at work being in the main two, viz. the dwindling of appendages as mere organs of locomotion, and the conversion of a ventral groove into a tube.
I imagine that, among the Protostraca, forms were found somewhat resembling trilobites with markedly polychætan affinities; which, like Apus, possessed a deep ventral groove from one end of the body to the other, and also pleural fringes, as in many trilobites. This might be called the Trilobite stage (Fig. [167], A).
This groove became converted into a tube and so gave rise to the notochord, while the appendages were still free and the pleuræ had not met to form a new ventral surface. This might be called the Chordate Trilobite stage (Fig. [167], B).
Then, passing from the protostracan to the palæostracan stage, the oral and respiratory chambers were formed, not communicating with each other, in the manner described in previous chapters, a ventral groove in the metasomatic region being the only connection between respiratory chamber and cloaca. This might be called the Chordate Palæostracan stage (Fig. [167], C).
Fig. 167.—A, Diagram of Section through a Trilobite-like Animal; B, Diagram to illustrate the Suggested Formation of the Notochord from a Ventral Groove; C, Diagram to illustrate the Suggested Formation of the Post-Branchial Gut by the continuation of the same process of Ventral Groove-Formation, combined with Obliteration of Appendages and Growth of Pleural Folds; D, Diagram to illustrate the Completion of the Vertebrate Type by the Meeting of the Pleural Folds in the Mid-Ventral Line with the Obliteration of the Atrial Cavity and the Conversion of the Ventral Groove into the closed Alimentary Canal.
Al., alimentary canal; N., nervous system; My., myotome; Pl., pleuron; App., appendage; Neph., nephrocœle; Met., metacœle; Sd., segmental duct; Mes., mesonephros; At., atrial chamber; Nc., notochord; H., heart; F., fat body; Ng., notochordal groove. (These diagrams are intended to complete the diagrams on p. [413], which, as stated there, were purposely left incomplete.)
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 notochordal 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 Cephalaspidæ, Ammocœtes, or Amphioxus.