contrast between the two animals may be that
we have here an instance of an interesting gradation in evolution. From
serving primitively as the essential organ of the cleft the tongue-bar may
have undergone reduction and modification, becoming a secondary bar in
_Amphioxus_, subordinate to the primary bars in size, vascularity and
development; finally, in the craniate vertebrates it would then have
completed its involution, the suggestion having been made that the
tongue-bars are represented by the thymus-primordia.

_Gill-pouches and Gill-pores_.--Only rarely do the gill-slits open freely
and directly to the exterior (fig. 1). In most species of _Balanoglossus_
each gill-slit may be said to open into its own atrial chamber or
gill-pouch; this in its turn opens to the exterior by a minute gill-pore.
There are, therefore, as many gill-pouches as there are gill-slits and as
many gill-pores as pouches. The gill-pores occur on each side of the dorsal
aspect of the worm in a longitudinal series at the base of a shallow
groove, the branchial groove. The respiratory current of water is therefore
conducted to the exterior by different means from that adopted by
_Amphioxus_, and this difference is so great that the theory which seeks to
explain it has to postulate radical changes of structure, function and
topography.

_Excretory and Vascular Systems_.--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. The vascular system itself is quite peculiar, consisting
of lacunae and channels destitute of endothelium, situated within the
thickness of the basement-membrane of the body-wall, of the gut-wall and of
the mesenteries. The blood, which is a non-corpuscular fluid, is propelled
forwards by the contractile dorsal vessel and collected into the central
blood-sinus; this lies over the stomochord, and is surrounded on three
sides by a closed vesicle, with contractile walls, called the pericardium
(_Herzblase_). By the pulsation of the pericardial vesicle (best observed
in the larva) the blood is driven into the glomeru