The enigmatic _neurenteric canal_ is a very old embryonic organ, and of great phylogenetic interest, because it arises in the same way in all the chordonia (both tunicates and vertebrates). In every case it touches or embraces like an arch the posterior end of the chorda, which has been developed here in front out of the middle line of the primitive gut (between the two cœlom-folds of the sickle groove) (“head-process,” Fig. 123 _kf_). These very ancient and strictly hereditary structures, which have no physiological significance to-day, deserve (as “rudimentary organs”) our closest attention. The tenacity with which the useless neurenteric canal has been transmitted down to man through the whole series of vertebrates is of equal interest for the theory of descent in general, and the phylogeny of the chordonia in particular.
The connection which the neurenteric canal (Fig. 123 _cn_) establishes between the dorsal nerve-tube (_n_) and the ventral gut-tube (_d_) is seen very plainly in the amphioxus in a longitudinal section of the cœlomula, as soon as the primitive mouth is completely closed at its hinder end. The medullary tube has still at this stage an opening at the forward end, the neuroporus Fig. 83 _np_). This opening also is afterwards closed. There are then two completely closed canals over each other—the medullary tube above and the gastric tube below, the two being separated by the chorda. The same features as in the acrania are exhibited by the related tunicates, the ascidiæ.
Fig.125. Longitudinal section of the chordula of a frog. Fig. 125—Longitudinal section of the chordula of a frog. (From _Balfour._) _nc_ nerve-tube, _x_ canalis neurentericus, _al_ alimentary canal, _yk_ yelk-cells, _m_ mesoderm.
Again, we find the neurenteric canal in just the same form and situation in the amphibia. A longitudinal section of a young tadpole (Fig. 125) shows how we may penetrate from the still open primitive mouth (_x_) either into the wide primitive gut-cavity (_al_) or the narrow overlying nerve-tube. A little later, when the primitive mouth is closed, the narrow neurenteric canal (Fig. 126 _ne_) represents the arched connection between the dorsal medullary canal (_mc_) and the ventral gastric canal.