But only slight modifications are required to produce the Tornaria larva of the Enteropneusta and other larvae, including the special type that is inferred from the Dipleurula larval stages of recent forms to have characterized the ancestor of the Echinoderms. We cannot enter here into all the details of comparison between these larval forms; amid much that is hypothetical a few homologies are widely accepted, and the preceding account will show the kind of relation that the Echinoderms bear to other animals, including what are now usually regarded as the ancestors of the Chordata (to which back-boned animals belong), as well as the nature of the evidence that their study has been, or may be, made to yield.
One of the most singular facts concerning the geographical distribution of Enteropneusta has recently been brought to light by Benham, who found a species of Balanoglossus, sensu stricto, on the coast of New Zealand hardly distinguishable from one occurring off Japan.
In accordance with this view there would be also some probability in favour of regarding the collar nerve-tube of the Enteropneusta as the equivalent of the cerebral vesicle only of Amphioxus and the Ascidian tadpole, and also of the primary forebrain of vertebrates.
In some of these respects the Dipleurula may have diverged from the ancestor of Enteropneusta and of other animals, but it could not as yet have been recognized as echinodermal by a zoologist, for it presented none of the structural peculiarities of the modern adult echinoderm.
In spite of the criticisms which have been made on the conclusion that Phoronis is allied to the Pterobranchia, it is thus possible that the view is a sound one, and that the Phoronidea should take their place, with the Enteropneusta and the Pterobranchia, as an order of the Hemichordata.
The affinity of the Pterobranchia to the Enteropneusta may be regarded as definitely established.
It is within the bounds of possibility that Tornaria actually does indicate a remote affinity on the part of the Enteropneusta to the Echinoderms, not only on account of its external form, but also by reason of the possession of a dorsal water-pore communicating with the anterior body-cavity.
It seems likely that the coelomic pore-canals were originally excretory organs, but in the existing Enteropneusta the pore-canals (especially the collar canals) have, as we have seen, acquired new functions or become vestigial, and the function of excretion is now mainly accomplished by a structure peculiar to the Enteropneusta called the glomerulus, a vascular complex placed on either side of the anterior portion of the stomochord, projecting into the proboscis-coelom.
In 1883-1886 Bateson showed by his embryological researches that the Enteropneusta exhibit chordate (vertebrate) affinities in respect of the coelomic, skeletal and nervous systems as well as in regard to the respiratory system, and, further, that the gill-slits are formed upon a plan similar to that of the gillslits of Amphioxus, being subdivided by tongue-bars which depend from the dorsal borders of the slits.
K, the hollow central nervous system of some Enteropneusta and of Vertebrates.