proboscis-pores are highly variable, and frequently only one is present,
that on the left side; sometimes the pore-canals of the proboscis unite to
open by a common median orifice, and sometimes their communication with the
proboscis-coelom appears to be occluded, and finally the pore-canals may be
quite vestigial. The collar-pores are remarkable for their constancy; this
is probably owing co the fact that they have become adapted to a special
function, the inhalation of water to render the collar turgid during
progression. There are reasons for supposing that the truncal coelom was at
one time provided with pore-canals, but supposed vestiges of these
structures have only been described for one genus, _Spengelia_, in which
they lie near the anterior end of the truncal coelom.

_Enteron_.--Not only is the coelom thus subdivided, but the enteron (gut,
alimentary canal, digestive tube) itself shows indications of three main
subsections in continuity with one another:--(1) proboscis-gut
(_Eicheldarm_, stomochord, _vide infra_); (2) collar-gut (buccal cavity,
throat); (3) truncal gut extending from the collar to the vent.

_Stomochord_.--The proboscis-gut occurs as an outgrowth from the anterior
dorsal wall of the collar-gut, and extends forward into the basal
(posterior) region of the proboscis, through the neck into the
proboscis-coelom, ending blindly in front. Although an integral portion of
the gut, it has ceased to assist in alimentation, its epithelium undergoes
vacuolar differentiation and hypertrophy, and its lumen becomes more or
less vestigial. It has, in fact, become metamorphosed into a resistant
supporting structure resembling in some respects the notochord of the true
Chordata, but probably not directly comparable with the latter structure,
being related to it solely by way of substitution. On account of the
presence and mode of origin (from the gut-wall) of this organ Bateson
introduced the term hemichorda as a phyletic name for the class
Enteropneusta. As the proboscis-gut appears to have undoubtedly skeletal
properties, and as it also has topographical relations with the mouth, it
has been designated in English by the non-committal term stomochord. It is
not a simple diverticulum of the collar-gut, but a complex structure
possessing paired lateral pouches and a ventral convexity (ventral caecum)
which rests in a concavity at the front end of the body of the nuchal
skeleton (fig. 3). In some species (_Spengel